EQUITY OFFICE PROPERTIES TRUST
S-4, 2000-03-30
REAL ESTATE INVESTMENT TRUSTS
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<PAGE>   1

     AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON MARCH 30, 2000
                                                     REGISTRATION NO. 333-
- --------------------------------------------------------------------------------
- --------------------------------------------------------------------------------
                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549
                         ------------------------------
                                    FORM S-4
                             REGISTRATION STATEMENT
                                     UNDER
                           THE SECURITIES ACT OF 1933
                         ------------------------------
                         EQUITY OFFICE PROPERTIES TRUST
             (EXACT NAME OF REGISTRANT AS SPECIFIED IN ITS CHARTER)

<TABLE>
<S>                                  <C>                                  <C>
              MARYLAND                               6798                              36-4151656
    (STATE OR OTHER JURISDICTION         (PRIMARY STANDARD INDUSTRIAL               (I.R.S. EMPLOYER
 OF INCORPORATION OR ORGANIZATION)       CLASSIFICATION CODE NUMBER)              IDENTIFICATION NO.)
</TABLE>

                         ------------------------------
                        TWO NORTH RIVERSIDE, SUITE 2100
                            CHICAGO, ILLINOIS 60606
                                 (312) 466-3300
  (ADDRESS, INCLUDING ZIP CODE, AND TELEPHONE NUMBER, INCLUDING AREA CODE, OF
                   REGISTRANT'S PRINCIPAL EXECUTIVE OFFICES)

                            STANLEY M. STEVENS, ESQ.
                EXECUTIVE VICE PRESIDENT AND CHIEF LEGAL COUNSEL
                         EQUITY OFFICE PROPERTIES TRUST
                     TWO NORTH RIVERSIDE PLAZA, SUITE 2100
                            CHICAGO, ILLINOIS 60606
                                 (312) 466-3300
 (NAME, ADDRESS, INCLUDING ZIP CODE, AND TELEPHONE NUMBER, INCLUDING AREA CODE,
                             OF AGENT FOR SERVICE)
                         ------------------------------
                                   COPIES TO:

<TABLE>
<S>                                                   <C>
            J. WARREN GORRELL, JR., ESQ.                             WILLIAM B. FRYER, ESQ.
              GEORGE P. BARSNESS, ESQ.                              JOHN J. KELLEY III, ESQ.
               HOGAN & HARTSON L.L.P.                                    KING & SPALDING
             555 THIRTEENTH STREET, N.W.                              191 PEACHTREE STREET
             WASHINGTON, D.C. 20004-1109                             ATLANTA, GA 30303-1763
                   (202) 637-5600                                        (404) 572-4600
</TABLE>

                         ------------------------------
     APPROXIMATE DATE OF COMMENCEMENT OF PROPOSED SALE OF THE SECURITIES TO THE
PUBLIC: AS SOON AS PRACTICABLE AFTER THE EFFECTIVE DATE OF THIS REGISTRATION
STATEMENT.
     If the securities being registered on this form are being offered in
connection with the formation of a holding company and there is compliance with
General Instruction G, check the following box.  [ ]
     If this Form is filed to register additional securities for an offering
pursuant to Rule 462(b) under the Securities Act, check the following box and
list the Securities Act registration statement number of the earlier effective
registration statement for the same offering.  [ ]
     If this Form is a post-effective amendment filed pursuant to Rule 462(d)
under the Securities Act, check the following box and list the Securities Act
registration statement number of the earlier effective registration statement
for the same offering.  [ ]
                         ------------------------------
                        CALCULATION OF REGISTRATION FEE

<TABLE>
<CAPTION>
- ---------------------------------------------------------------------------------------------------------------------------------
- ---------------------------------------------------------------------------------------------------------------------------------
     TITLE OF EACH CLASS OF                                   PROPOSED MAXIMUM        PROPOSED MAXIMUM
        SECURITIES TO BE                AMOUNT TO BE         OFFERING PRICE PER      AGGREGATE OFFERING          AMOUNT OF
           REGISTERED                  REGISTERED(1)            COMMON SHARE               PRICE            REGISTRATION FEE(2)
- ---------------------------------------------------------------------------------------------------------------------------------
<S>                                <C>                     <C>                     <C>                     <C>
Common shares of beneficial
  interest, par value $.01 per
  share..........................        70,387,333            Not Applicable          Not Applicable           $445,738.68
- ---------------------------------------------------------------------------------------------------------------------------------
- ---------------------------------------------------------------------------------------------------------------------------------
</TABLE>

(1) Represents the maximum number of common shares of beneficial interest, par
    value $.01 per share, of Equity Office Properties Trust that may be issued
    pursuant to the transactions described herein, based on (a) 158,975,741
    shares of common stock with no par value of Cornerstone Properties Inc.,
    which is the maximum number of shares of Cornerstone common stock that may
    be outstanding immediately before the consummation of the transactions
    described herein, assuming exercise of all outstanding options and warrants
    to purchase shares of Cornerstone common stock, whether or not currently
    exercisable, conversion of all units of limited partnership interest in
    Cornerstone Properties Limited Partnership that are not held by Cornerstone
    into shares of Cornerstone common stock, conversion of all Cornerstone 7%
    preferred stock into shares of Cornerstone common stock and conversion of
    all outstanding convertible promissory notes of Cornerstone into Cornerstone
    common stock, (b) conversion of 58,551,525 shares of Cornerstone common
    stock in the merger into the right to receive cash under the terms of the
    merger agreement, and (c) the exchange ratio of 0.7009 of an Equity Office
    common share for each outstanding share of Cornerstone common stock in
    excess of the 58,551,525 shares of Cornerstone common stock converted in the
    merger into the right to receive cash.
(2) Pursuant to Rule 457(f)(1) and 457(c) under the Securities Act of 1933, the
    registration fee has been calculated based on a price of $17.25 per share of
    Cornerstone common stock, which equals the average of the high and low price
    per share of Cornerstone common stock as reported on the New York Stock
    Exchange on March 24, 2000.
                         ------------------------------
     THE REGISTRANT HEREBY AMENDS THIS REGISTRATION STATEMENT ON SUCH DATE OR
DATES AS MAY BE NECESSARY TO DELAY ITS EFFECTIVE DATE UNTIL THE REGISTRANT SHALL
FILE A FURTHER AMENDMENT WHICH SPECIFICALLY STATES THAT THIS REGISTRATION
STATEMENT SHALL THEREAFTER BECOME EFFECTIVE IN ACCORDANCE WITH SECTION 8(a) OF
THE SECURITIES ACT OF 1933 OR UNTIL THE REGISTRATION STATEMENT SHALL BECOME
EFFECTIVE ON SUCH DATE AS THE COMMISSION, ACTING PURSUANT TO SAID SECTION 8(a)
MAY DETERMINE.
- --------------------------------------------------------------------------------
- --------------------------------------------------------------------------------
<PAGE>   2

   THE INFORMATION IN THIS JOINT PROXY STATEMENT/PROSPECTUS IS NOT COMPLETE AND
   MAY BE CHANGED. WE MAY NOT SELL THESE SECURITIES UNTIL THE REGISTRATION
   STATEMENT FILED WITH THE SEC IS EFFECTIVE.
   THIS JOINT PROXY STATEMENT/PROSPECTUS IS NOT AN OFFER TO SELL THESE
   SECURITIES AND IS NOT SOLICITING AN OFFER TO BUY THESE SECURITIES IN ANY
   JURISDICTION WHERE THE OFFER OR SALE IS NOT PERMITTED.

[EQUITY OFFICE LOGO]                                          [CORNERSTONE LOGO]

                 MERGER PROPOSED -- YOUR VOTE IS VERY IMPORTANT

    The boards of both companies have approved a merger agreement that provides
for the acquisition of Cornerstone by Equity Office. We believe the merger will
provide the combined company with high quality office buildings in locations
that are complementary to existing Equity Office geographic locations. In
addition, by increasing the office space owned and operated by Equity Office by
approximately 24% based on square footage, the merger will solidify Equity
Office's position as the nation's largest publicly-traded owner and operator of
office properties.

    In the merger, Cornerstone common stockholders may elect to receive for each
share of common stock either $18.00 in cash, without interest, or 0.7009 of an
Equity Office common share, subject to proration if the cash election is
oversubscribed or undersubscribed. The value of Equity Office common shares to
be received by Cornerstone common stockholders will depend upon their market
price at the time of the merger. Equity Office common shares are traded on the
New York Stock Exchange under the symbol "EOP." On      , 2000, Equity Office
common shares closed at $    per share. Based on the number of Equity Office and
Cornerstone common shares outstanding as of the date of this joint proxy
statement/prospectus, we estimate that, upon completion of the merger,
approximately     % of the outstanding Equity Office common shares will be owned
by Cornerstone stockholders and approximately     % will be owned by current
Equity Office shareholders. Holders of Cornerstone preferred stock will receive
$18.00 in cash per share, without interest, together with accrued but unpaid
dividends through the closing of the merger.

    AFTER CAREFUL CONSIDERATION, THE BOARDS OF EQUITY OFFICE AND CORNERSTONE
HAVE DETERMINED THAT THE MERGER IS IN THE BEST INTERESTS OF THEIR RESPECTIVE
SHAREHOLDERS, AND EACH BOARD RECOMMENDS YOUR VOTING FOR APPROVAL OF THE MERGER
AGREEMENT AND THE MERGER.

    We cannot complete the merger unless the shareholders of both companies
approve the merger and the merger agreement. Equity Office will hold a special
meeting of its common shareholders and Cornerstone will hold a special meeting
of its common and preferred stockholders to vote on these matters. Whether or
not you plan to attend your special meeting, please take the time to vote by
completing and mailing the enclosed proxy card or, if you are an Equity Office
common shareholder, you may instead vote by telephone or through the Internet as
instructed on your proxy card.

    This document provides you with detailed information about these meetings
and the proposed merger. You can also get information from publicly available
documents filed by both companies with the SEC. WE ENCOURAGE YOU TO READ THIS
ENTIRE DOCUMENT CAREFULLY, INCLUDING THE SECTION ENTITLED "RISK FACTORS"
BEGINNING ON PAGE 13.

    The dates, times and places of the meetings are as follows:

<TABLE>
<S>                                                      <C>
            For EQUITY OFFICE Shareholders:                           For CORNERSTONE Stockholders:
              , 2000 at   :00 a.m., local time                      , 2000 at   :00 a.m., local time
               [Hotel/Conference Center]                                [Hotel/Conference Center]
                   [Street Address]                                         [Street Address]
                   Chicago, Illinois                                       New York, New York

                      Samuel Zell                                          William Wilson III
                 Chairman of the Board                                    Chairman of the Board
            Equity Office Properties Trust                             Cornerstone Properties Inc.
</TABLE>

   EACH VOTE IS IMPORTANT. PLEASE COMPLETE, SIGN, DATE AND RETURN YOUR PROXY.

Neither the SEC nor any state securities commission has approved or disapproved
of the securities to be issued in the merger or passed upon the adequacy or
accuracy of this joint proxy statement/prospectus. Any representation to the
contrary is a criminal offense.

           This joint proxy statement/prospectus is dated      , 2000
             and it is first being mailed on or about      , 2000.
<PAGE>   3

                         EQUITY OFFICE PROPERTIES TRUST
                           TWO NORTH RIVERSIDE PLAZA
                                   SUITE 2100
                            CHICAGO, ILLINOIS 60606

                   NOTICE OF SPECIAL MEETING OF SHAREHOLDERS
                        TO BE HELD ON             , 2000

     A special meeting of shareholders of Equity Office Properties Trust, a
Maryland real estate investment trust, will be held at   :00 a.m., local time,
on             , 2000, at                , for the following purposes:

     1. To consider and vote on the adoption and approval of the agreement and
        plan of merger, dated as of February 11, 2000, by and among Equity
        Office, EOP Operating Limited Partnership, a Delaware limited
        partnership, Cornerstone Properties Inc., a Nevada corporation, and
        Cornerstone Properties Limited Partnership, a Delaware limited
        partnership, and the merger of Cornerstone with and into Equity Office
        under the merger agreement.

     2. To transact such other business as may properly come before the special
        meeting or any adjournments or postponements.

     Only holders of record of Equity Office common shares at the close of
business on             , 2000 are entitled to notice of, and to vote at, the
special meeting and any adjournments or postponements.

     It is important that your common shares be represented and voted at the
meeting. If you do not plan to attend the meeting and vote your common shares in
person, please vote in one of these ways:

     - USE THE TOLL-FREE TELEPHONE NUMBER shown on your proxy card (this call is
       free in the U.S. and Canada);

     - VISIT THE WEBSITE address shown on your proxy card to vote via the
       Internet; or

     - MARK, SIGN, DATE AND PROMPTLY RETURN your enclosed proxy card in the
       postage-paid envelope.

     Any proxy may be revoked at any time before its exercise at the meeting.

                                          By order of Equity Office's
                                          board of trustees,

                                          Stanley M. Stevens
                                          Executive Vice President
                                          and Chief Legal Counsel

            , 2000

                 MERGER PROPOSED -- YOUR VOTE IS VERY IMPORTANT

     THE EQUITY OFFICE BOARD OF TRUSTEES HAS APPROVED THE MERGER AGREEMENT AND
THE MERGER OF CORNERSTONE INTO EQUITY OFFICE AND RECOMMENDS THAT YOU VOTE TO
APPROVE THE MERGER AGREEMENT AND THE MERGER.
<PAGE>   4

                                 VOTING METHODS

     YOU HAVE THE RIGHT TO VOTE AND, IF DESIRED, TO REVOKE YOUR PROXY ANY TIME
BEFORE THE EQUITY OFFICE SPECIAL MEETING.

<TABLE>
    <S>                                     <C>                                    <C>

    U.S. Mail                               1. Mark your selections
    [MAIL BOX GRAPHIC]                      2. Date and sign your name
    Proxy Card Voting                          exactly as it appears on your
                                               proxy card
                                            3. Mail to EquiServe in the
                                               return envelope
</TABLE>

<TABLE>
    <S>                                     <C>                                    <C>
                                            1. Within the U.S. and Canada                  /X/
    [TELEPHONE GRAPHIC]                        dial [1-877-779-8683] and                  VOTE
    Telephone Voting                           outside                                  EquiServe
                                               the U.S. and Canada call                   Vote
                                               collect                                 Tabulation
                                               [1-201-536-8073]
                                            2. Enter your control number
                                               (printed on your proxy card
                                               above your name)
                                            3. Follow the recorded
                                               instructions
</TABLE>

<TABLE>
    <S>                                     <C>                                    <C>
                                            1. Go to website
    [COMPUTER GRAPHIC]                         http://www.eproxyvote.com/eop
    Internet Voting                         2. Enter your 14 digit control
                                               number (printed on your proxy
                                               card above your name)
                                            3. Follow the instructions
                                               provided
</TABLE>
<PAGE>   5

                   NOTICE OF SPECIAL MEETING OF STOCKHOLDERS
                        TO BE HELD ON             , 2000
                          CORNERSTONE PROPERTIES INC.
                              126 EAST 56TH STREET
                            NEW YORK, NEW YORK 10022

                                                                          , 2000

     A special meeting of stockholders of Cornerstone Properties Inc. will be
held at      :00 a.m., local time, on             , 2000, at                .

     Your board of directors asks you to attend this meeting, in person or by
proxy, for the following purposes:

     1. To consider and vote upon a proposal to approve the agreement and plan
        of merger, dated as of February 11, 2000, by and among Cornerstone,
        Cornerstone Properties Limited Partnership, Equity Office Properties
        Trust and EOP Operating Limited Partnership, a copy of which is attached
        as Annex A to the accompanying joint proxy statement/prospectus, and the
        merger of Cornerstone with and into Equity Office.

     2. To transact such other business as may properly come before the special
        meeting or any adjournments or postponements.

     Only holders of record of Cornerstone common stock and 7% cumulative
convertible preferred stock at the close of business on             , 2000 are
entitled to notice of, and to vote at, the special meeting or any adjournments
or postponements.

     We invite you to attend the special meeting because it is important that
your shares be represented at the meeting. Whether or not you plan to attend the
special meeting, please sign, date and return the enclosed proxy card in the
accompanying envelope. If you attend the meeting, you may vote in person, which
will revoke a signed proxy if that has already been sent. You may also revoke
your proxy at any time before the meeting either in writing or by notifying us.

Sincerely,

<TABLE>
<S>                                                      <C>
William Wilson III                                       John S. Moody
Chairman of the Board                                    Chief Executive Officer and President
</TABLE>

                 MERGER PROPOSED -- YOUR VOTE IS VERY IMPORTANT

     THE CORNERSTONE BOARD OF DIRECTORS HAS APPROVED THE MERGER OF CORNERSTONE
INTO EQUITY OFFICE AND RECOMMENDS THAT YOU VOTE TO APPROVE THE MERGER AGREEMENT.
<PAGE>   6

                               TABLE OF CONTENTS

<TABLE>
<CAPTION>
                                        PAGE
                                        ----
<S>                                     <C>
SUMMARY...............................    1
  The Companies.......................    1
  The Equity Office Special Meeting;
     Vote Required....................    1
  Equity Office Voting Agreements.....    1
  The Cornerstone Special Meeting;
     Vote Required....................    2
  Cornerstone Voting Agreements.......    2
  Recommendation of Equity Office
     Board............................    2
  Recommendation of Cornerstone
     Board............................    2
  Fairness Opinions of Financial
     Advisors.........................    2
  The Merger..........................    3
  Selected Historical Consolidated
     Financial Data...................    5
  Equity Office.......................    5
  Cornerstone.........................    8
  Equivalent Per Share Data...........    8
  Market Prices of Common Shares......   10
  Summary Unaudited Pro Forma
     Condensed Combined Financial
     Data.............................   11
RISK FACTORS..........................   13
A WARNING ABOUT FORWARD-LOOKING
  STATEMENTS..........................   18
THE EQUITY OFFICE SPECIAL MEETING.....   19
  Date, Time, Place and Purpose of the
     Equity Office Special Meeting....   19
  Who Can Vote........................   19
  Voting by Proxy Holders.............   19
  Vote by Telephone...................   19
  Vote by Internet....................   19
  Vote by Mail........................   20
  Required Vote.......................   20
  Voting on Other Matters.............   20
  How You May Revoke Your Proxy
     Instructions.....................   20
  How Votes Are Counted...............   20
  Cost of This Proxy Statement........   20
  Attending the Equity Office Special
     Meeting..........................   20
  List of Equity Office Common
     Shareholders.....................   21
THE CORNERSTONE SPECIAL MEETING.......   22
  Date, Time, Place and Purpose of the
     Cornerstone Special Meeting......   22
  Who Can Vote........................   22
  Voting by Proxy Holders.............   22
  Voting and Election Procedures......   22
  Required Vote.......................   23
  Voting on Other Matters.............   23
  How You May Revoke Your Proxy
     Instructions or Form of
     Election.........................   23
</TABLE>

<TABLE>
<CAPTION>
                                        PAGE
                                        ----
<S>                                     <C>
  How Votes Are Counted...............   23
  Cost of This Proxy Solicitation.....   24
  Attending the Cornerstone Special
     Meeting..........................   24
  List of Cornerstone Stockholders....   24
THE MERGER............................   25
  Structure of the Mergers............   25
  Background of the Merger............   25
  Equity Office's Reasons for the
     Merger; Recommendation of the
     Equity Office Board..............   29
  Opinion of Equity Office's Financial
     Advisor..........................   30
  Cornerstone's Reasons for the
     Merger; Recommendation of the
     Cornerstone Board................   35
  Opinion of Cornerstone's Financial
     Advisor..........................   37
  Equity Office Board of Trustees and
     Executive Officers after the
     Merger...........................   41
  Interests of Cornerstone's
     Directors, Officers and
     Significant Stockholders in the
     Merger...........................   42
  The Partnership Merger..............   44
  Merger Financing....................   45
  Material U.S. Federal Income Tax
     Consequences Relating to the
     Merger...........................   45
  Federal Income Tax Consequences of
     the Merger to Cornerstone,
     Cornerstone Stockholders, Equity
     Office and Equity Office
     Shareholders.....................   46
  Pre-Merger Dividend.................   50
  REIT Qualification..................   50
  U.S. Taxation of Sales of Equity
     Office Common Shares by
     Non-U.S Holders..................   52
  Amendments to Equity Office's
     Declaration of Trust Relating to
     "Domestically-Controlled" REIT
     Status...........................   53
  Accounting Treatment................   53
  Restrictions on Resales by
     Affiliates.......................   53
  No Dissenters' Rights...............   54
THE MERGER AGREEMENT..................   55
  Closing; Effective Time of the
     Merger...........................   55
  Merger Consideration................   55
  Proration of Cash and Equity Office
  Common Shares Issued in the
  Merger..............................   55
  Surrender of Cornerstone
     Certificates.....................   56
  Treatment of Cornerstone Stock
     Options..........................   57
  Representations and Warranties of
     Equity Office and Cornerstone....   57
</TABLE>

                                        i
<PAGE>   7

<TABLE>
<CAPTION>
                                        PAGE
                                        ----
<S>                                     <C>
  Conduct of Business of Equity Office
     and EOP Partnership Pending
     the Merger.......................   58
  Conduct of Business of Cornerstone
     and Cornerstone Partnership
     Pending the Merger...............   59
  Conditions to the Merger and the
     Partnership Merger...............   62
  No Solicitation by Cornerstone......   64
  Termination of the Merger
     Agreement........................   66
  Waiver and Amendment of the Merger
     Agreement........................   69
  Indemnification; Directors' and
     Officers' Insurance..............   70
  Assumption of Cornerstone's
     Obligations under Registration
     Rights Agreements................   70
  Voting Agreements...................   72
  Preferred Stock Option Agreement....   73
  Acquisition of WCP Services.........   73
COMPARISON OF SHAREHOLDER RIGHTS......   74
  Authorized Shares...................   74
  Voting Rights.......................   75
  Classification of the Board.........   75
  Number of Trustees/Directors;
     Removal of Trustees/Directors;
     Vacancies........................   75
  Limitation of Trustee/Director and
     Officer Liability................   76
  Indemnification.....................   76
  Duties of Trustees and Directors....   79
  Maryland Elective Provisions........   80
  Call of Special Meetings of
     Shareholders.....................   81
  Shareholder Action by Written
     Consent..........................   81
  Advance Notice Provisions for
     Shareholder Nominations and
     Shareholder New Business
     Proposals........................   81
</TABLE>

<TABLE>
<CAPTION>
                                        PAGE
                                        ----
<S>                                     <C>
  Amendment of the Declaration of
     Trust and Articles of
     Incorporation....................   81
  Amendment of the Bylaws.............   82
  Mergers, Consolidations and Sales of
     Assets...........................   83
  Dissolution of Equity Office or
     Cornerstone; Termination of REIT
     Status...........................   84
  Business Combinations with
     Interested Shareholders..........   84
  Control Share Acquisitions..........   84
  Other Constituencies................   85
  Dissenters' Rights..................   85
  Distributions.......................   86
  REIT Ownership Limitations..........   86
LEGAL MATTERS.........................   88
EXPERTS...............................   89
SHAREHOLDER PROPOSALS.................   89
OTHER MATTERS.........................   89
WHERE YOU CAN FIND MORE INFORMATION...   90
WHAT INFORMATION YOU SHOULD RELY ON...   91
PRO FORMA CONDENSED CONSOLIDATED
  FINANCIAL STATEMENTS................  F-1
Annex A -- Agreement and Plan
  of Merger...........................  A-1
Annex B -- Opinion of J.P. Morgan
  Securities Inc., dated February 11,
  2000................................  B-1
Annex C -- Opinion of Lazard Freres &
  Co. LLC, dated February 11, 2000....  C-1
Annex D -- Amendments to Equity Office
  Declaration of Trust to be Effected
  as Part of the Merger...............  D-1
</TABLE>

                                       ii
<PAGE>   8

                      QUESTIONS & ANSWERS ABOUT THE MERGER

Q: WHY ARE EQUITY OFFICE AND CORNERSTONE PROPOSING THE MERGER?

A: Both Equity Office and Cornerstone believe the merger will provide the
   combined company with high quality office buildings in locations that are
   complementary to existing Equity Office geographic locations. In addition, by
   increasing the office space owned and operated by Equity Office by
   approximately 24% based on square footage, the merger will solidify its
   position as the nation's largest publicly-traded owner and operator of office
   properties. The merger also is expected to enhance the combined company's
   ability to offer its customers a full range of office space options, as well
   as provide Equity Office with a development pipeline of new office properties
   in a supply-constrained market. To review the background and reasons for the
   merger in greater detail, see pages 25 through 41 of this document.

Q: WHAT WILL I RECEIVE IN THE MERGER?

A:Cornerstone Stockholders.  For all or a portion of its Cornerstone common
  stock, a Cornerstone common stockholder may elect to receive either $18.00 per
  share in cash, without interest, or 0.7009 of a common share, par value $.01
  per share, of Equity Office, subject to proration if the cash election is
  oversubscribed or undersubscribed.

  In the merger, holders of Cornerstone preferred stock will receive $18.00 per
  share in cash, without interest, together with accrued and unpaid dividends
  through the date of the merger closing.

  Equity Office Shareholders.  Each Equity Office common share or preferred
  share held by Equity Office shareholders will continue to represent one Equity
  Office common share or preferred share after the merger. Based on the number
  of Equity Office and Cornerstone common shares outstanding as of the date of
  this document, we estimate that, upon completion of the merger, approximately
       % of the outstanding Equity Office common shares will be owned by
  Cornerstone stockholders and approximately      % will be owned by current
  Equity Office shareholders.

Q: HOW WILL PRORATION AFFECT ME IF I OWN CORNERSTONE COMMON STOCK?

A: A total of 58,551,525 shares of Cornerstone common stock will be converted in
   the merger into the right to receive $18.00 per share in cash, without
   interest, and the number of shares of Cornerstone common stock issued and
   outstanding immediately before the merger in excess of 58,551,525 shares will
   be converted into the right to receive Equity Office common shares based on
   the 0.7009 exchange ratio.

   Because the aggregate cash portion of the merger consideration to be paid to
   Cornerstone's common stockholders is fixed at approximately $1.054 billion,
   or $18.00 times 58,551,525 shares, and because the number of Equity Office
   common shares issuable in the merger will equal 0.7009 times the number of
   shares of Cornerstone common stock issued and outstanding immediately before
   the closing of the merger in excess of 58,551,525 shares, the amount of cash
   and the number of Equity Office common shares that each Cornerstone common
   stockholder will receive in the merger may be subject to proration based on
   the elections made by other Cornerstone common stockholders.

    For example:

    Oversubscription of Cash

    - if cash elections are received for a number of shares of Cornerstone
      common stock which is greater than 58,551,525 shares, then:

      -- each share of Cornerstone common stock for which an election is not
         received and each share of Cornerstone common stock for which a share
         election is received will be converted in the merger into 0.7009 of an
         Equity Office common share; and

      -- the shares of Cornerstone common stock for which cash elections are
         received will be converted in the merger into $18.00 per share in cash,
         without interest, and Equity Office common shares based on the 0.7009
         exchange ratio, on a proportionate basis, so that the aggregate num-

                                       iii
<PAGE>   9

ber of shares of Cornerstone common stock converted into cash in the merger
equals 58,551,525 shares, with Equity Office common shares issued for the
remainder of the shares of Cornerstone common stock converted in the merger.

    Oversubscription of Shares

    - if share elections are received for a number of shares of Cornerstone
      common stock that is greater than (a) the number of shares of Cornerstone
      common stock issued and outstanding immediately before the merger less (b)
      the 58,551,525 shares to be converted into cash in the merger, then:

      -- each share of Cornerstone common stock for which an election is not
         received and each share of Cornerstone common stock for which a cash
         election is received will be converted in the merger into the right to
         receive $18.00 in cash, without interest; and

      -- the shares of Cornerstone common stock for which share elections are
         received will be converted in the merger into Equity Office common
         shares based on the 0.7009 exchange ratio, and $18.00 per share in
         cash, without interest, on a proportionate basis, so that the aggregate
         number of shares of Cornerstone common stock converted into cash in the
         merger equals 58,551,525 shares, with Equity Office common shares
         issued for the remainder of the shares of Cornerstone common stock
         converted in the merger.

    Undersubscription of Cash and Shares

    - if cash elections are received for less than the 58,551,525 shares of
      Cornerstone common stock to be converted in the merger into cash AND share
      elections are received for less than the maximum number of shares of
      Cornerstone common stock to be converted into Equity Office common shares
      in the merger, then:

      -- each share of Cornerstone common stock for which a cash election is
         received will be converted in the merger into the right to receive
         $18.00 in cash, without interest;

      -- each share of Cornerstone common stock for which a share election is
         received will be converted into 0.7009 of an Equity Office common
         share; and

      -- the shares of Cornerstone common stock for which no elections are
         received will be converted into cash and Equity Office common shares so
         that the aggregate number of shares of Cornerstone common stock
         converted into $18.00 in cash, without interest, in the merger equals
         58,551,525 shares, with Equity Office common shares based on the 0.7009
         exchange ratio issued for the remainder of the shares of Cornerstone
         common stock converted in the merger.

Q: HOW DO I ELECT THE TYPE OF CONSIDERATION I WOULD LIKE TO RECEIVE IN THE
   MERGER?

A: You should complete and sign the form of election included with this joint
   proxy statement/prospectus and return it and your Cornerstone common stock
   certificates to the exchange agent in accordance with the instructions in the
   form of election. The exchange agent must receive your properly completed and
   signed form of election and your certificates by 5:00 p.m., Eastern Time, on
             , 2000. If your Cornerstone common stock is held in "street name,"
   your broker will provide you with instructions as to how to complete and
   return the form of election.

Q: IS MY ELECTION IRREVOCABLE?

A: No. You may change your election or revoke it at any time before 5:00 p.m.,
   Eastern Time, on           , 2000, by submitting written notice to the
   exchange agent, who must receive the notice at or before that time.

Q: WHAT HAPPENS IF THE PRICE OF EQUITY OFFICE COMMON SHARES AND/OR CORNERSTONE
   COMMON STOCK CHANGES BEFORE THE CLOSING OF THE TRANSACTION?

A: No change will be made to the 0.7009 exchange ratio, or the $18.00 per share
   cash amount payable in the merger. The exchange ratio is fixed. Because the
   market value of Equity Office common shares will fluctuate before and after
   the closing of the merger, the value of any Equity Office common shares that
   you receive in the merger will fluctuate as

                                       iv
<PAGE>   10

   well, and may differ from $18.00 per share of Cornerstone common stock. You
   should obtain current market prices for Equity Office common shares and
   shares of Cornerstone common stock.

Q: HOW WAS THE EXCHANGE RATIO DETERMINED?

A: The exchange ratio was determined as part of the overall negotiation between
   Equity Office and Cornerstone regarding the merger consideration, taking into
   account the other terms of the transaction, including the aggregate and per
   share amounts of merger consideration payable in cash and the consideration
   payable to holders of Cornerstone preferred stock.

Q: WILL ANY FRACTIONAL EQUITY OFFICE COMMON SHARES BE ISSUED IN THE MERGER?

A: No. Cash will be paid instead of fractional shares. This cash will be in
   addition to the approximately $1.054 billion of cash required to be paid to
   holders of Cornerstone common stock as part of the merger consideration.

Q: WHAT AM I BEING ASKED TO VOTE UPON?

A: Cornerstone Stockholders.  You are being asked to approve the merger
   agreement, which provides for the merger of Cornerstone with and into Equity
   Office. Approval of the merger agreement requires the affirmative vote of at
   least a majority of the outstanding shares of Cornerstone common stock and at
   least two thirds of the outstanding shares of Cornerstone 7% cumulative
   convertible preferred stock.

   THE CORNERSTONE BOARD HAS APPROVED AND ADOPTED THE MERGER AGREEMENT AND
   RECOMMENDS THAT CORNERSTONE STOCKHOLDERS VOTE FOR APPROVAL OF THE MERGER
   AGREEMENT.

   Equity Office Shareholders.  You are being asked to approve the merger
   agreement and the merger. Approval of the proposal requires the affirmative
   vote of not less than a majority of all votes entitled to be cast.

   THE EQUITY OFFICE BOARD HAS APPROVED THE MERGER OF CORNERSTONE INTO EQUITY
   OFFICE AND RECOMMENDS THAT EQUITY OFFICE COMMON SHAREHOLDERS VOTE FOR
   APPROVAL OF THE MERGER AGREEMENT AND THE MERGER.

Q: DO CORNERSTONE COMMON OR PREFERRED STOCKHOLDERS HAVE DISSENTERS' RIGHTS?

A: No. Cornerstone is incorporated under Nevada law. Under Nevada law, because
   shares of Cornerstone common stock are listed on a national securities
   exchange and the holders of Cornerstone common stock will receive cash,
   Equity Office common shares or a combination of cash and Equity Office common
   shares, Cornerstone common stockholders have no rights to dissent and receive
   the appraised value of their shares in connection with the merger.
   Cornerstone preferred stockholders have dissenters' rights in the merger
   under Nevada law, but have irrevocably waived such rights.

Q: DO EQUITY OFFICE COMMON SHAREHOLDERS HAVE DISSENTERS' RIGHTS?

A: No. Following the merger, Equity Office shareholders will continue to own
   their Equity Office shares and, accordingly, will have no rights to an
   appraisal of their shares under Maryland law.

Q: HOW SOON AFTER THE SPECIAL MEETINGS WILL THE MERGER OCCUR?

A: If the merger is approved, we anticipate that the merger will occur as soon
   as practicable after the completion of both the Equity Office and the
   Cornerstone special meetings.

Q: WILL I RECOGNIZE TAXABLE GAIN OR LOSS AS A RESULT OF THE MERGER?

A: We expect the following tax consequences generally to apply:

   - Cornerstone common stockholders who exchange Cornerstone common stock
     solely for Equity Office common shares in the merger will not recognize
     gain or loss on the merger for U.S. federal income tax purposes.
     Cornerstone stockholders will, however, have to pay taxes on cash received
     for fractional shares.

    - Cornerstone common stockholders who receive solely cash in exchange for
      their shares, and holders of Cornerstone preferred stock, will recognize
      gain or loss for U.S. federal income tax purposes on the exchange of their
      shares of Cornerstone common or preferred stock.

                                        v
<PAGE>   11

    - Cornerstone common stockholders who receive both Equity Office common
      shares and cash will recognize gain up to the amount of cash received, but
      will not recognize any loss on the exchange.

    - A non-U.S. Cornerstone common stockholder that has held, actually and
      constructively, more than 5% of the outstanding Cornerstone common stock
      within the last five years will be fully taxable on the exchange of its
      Cornerstone common stock, regardless of whether it receives cash or Equity
      Office common shares or a combination of both in the exchange.

    Equity Office shareholders will not incur either gain or loss for U.S.
    federal income tax purposes as a result of the merger.

    THE TAX CONSEQUENCES TO CORNERSTONE STOCKHOLDERS WILL DEPEND ON YOUR
    PERSONAL SITUATION. YOU SHOULD CONSULT YOUR TAX ADVISOR FOR A FULL
    UNDERSTANDING OF THE TAX CONSEQUENCES OF THE MERGER TO YOU.

Q: WHAT WILL MY DIVIDENDS BE AFTER THE MERGER?

A: Until the merger is competed, Cornerstone stockholders will continue to
   receive regular dividends as authorized by Cornerstone's board of directors,
   including any dividends required to be paid immediately before the closing to
   maintain Cornerstone's status as a real estate investment trust, or REIT.
   After the completion of the merger, if you receive Equity Office common
   shares in the merger, you will receive the distributions payable to all
   holders of Equity Office common shares with a record date after the closing.
   Equity Office's current quarterly distributions are $0.42 per share per
   quarter. If you do not receive any Equity Office common shares in the merger,
   you will no longer receive dividends after the merger is completed.

Q: WHAT SHOULD I DO NOW?

A: Just indicate on your proxy card how you want to vote, and sign and mail it
   in the enclosed envelope as soon as possible so that your shares will be
   represented at your meeting. If you are a Cornerstone common stockholder, you
   should also send in your form of election as well as any share certificates.

   If you sign and send in your proxy and do not indicate how you want to vote,
   your proxy will be voted in favor of the proposal to approve the merger
   agreement, in the case of Cornerstone stockholders, and, in the case of
   Equity Office common shareholders, to approve the merger agreement and the
   merger. If you do not sign and send in your proxy or you abstain, it will
   have the effect of a vote against the merger.

Q: IF MY SHARES ARE HELD IN "STREET NAME" BY MY BROKER, WILL MY BROKER VOTE MY
   SHARES FOR ME?

A: Your broker will vote your Equity Office common shares or shares of
   Cornerstone common stock only if you provide instructions on how to vote. You
   should instruct your broker how to vote your shares, following the directions
   your broker provides. If you do not provide instructions to your broker, your
   shares will not be voted and this will have the effect of voting against the
   merger.

Q: SHOULD I SEND IN MY STOCK CERTIFICATES NOW?

A: If you are a Cornerstone common stockholder, you should complete and sign
   your form of election and return it and your Cornerstone common stock
   certificates to the exchange agent in accordance with the instructions in the
   form of election. If you do not send your form of election before the merger,
   or if you do not send your stock certificates to the exchange agent with your
   form of election, then following the merger, Equity Office will send to you
   written instructions for surrendering your stock certificates in order to
   receive the merger consideration. For shares held in "street name," your
   broker will provide you with instructions.

   If you are an Equity Office shareholder, you are not required to take any
   action regarding your Equity Office common or preferred share certificates.

Q: WHO CAN ANSWER MY QUESTIONS?

A: Equity Office Shareholders.  Equity Office common shareholders who have more
   questions about the merger or desire additional

                                       vi
<PAGE>   12

   copies of this joint proxy statement/prospectus or proxy cards should
   contact:

   Equity Office Properties Trust
   Two North Riverside Plaza,
   Suite 2100
   Chicago, Illinois 60606
   Attention: Diane Morefield
   Telephone: (312) 466-3286

   Cornerstone Stockholders.  Cornerstone common and preferred stockholders who
   have more questions about the merger or desire additional copies of this
   joint proxy statement/prospectus, proxy cards or, in the case of holders of
   Cornerstone common stock, forms of election should contact:

   Cornerstone Properties Inc.
   Tower 56
   126 East 56th Street, 6th Floor
   New York, New York 10022
   Attention: Thomas P. Loftus
   Telephone: (212) 605-7131

                                       vii
<PAGE>   13

                                    SUMMARY

     This summary highlights selected information from this joint proxy
statement/prospectus. It may not contain all of the detailed information that
may be important to you. To understand the merger fully and for a more complete
description of the legal terms of the merger, you should read carefully this
entire document and the other documents to which we refer, including the merger
agreement. For more information about Equity Office and Cornerstone, see "Where
You Can Find More Information" on page 90. Each item in this summary refers to
the pages where that subject is discussed more fully.

THE COMPANIES

EQUITY OFFICE PROPERTIES TRUST
Two North Riverside Plaza
Suite 2100
Chicago, Illinois 60606
(312) 466-3300

     Equity Office is the nation's largest publicly held owner and operator of
office properties, with a portfolio of 294 office buildings containing 77.0
million square feet in 23 states and the District of Columbia. Equity Office is
an independent real estate company that manages all aspects of its operations
internally. Equity Office owns all of its assets and conducts all of its
operations through EOP Operating Limited Partnership.

     Equity Office is the sole general partner of, and owns the controlling
interest in, EOP Partnership. Equity Office, through EOP Partnership, is engaged
in acquiring, owning, operating and leasing office properties and parking
facilities. Equity Office has elected to be taxed as a real estate investment
trust, or REIT, for federal income tax purposes. Equity Office, a Maryland real
estate investment trust, was organized in 1996 and began operations in 1997 to
continue and expand the national office property business of Mr. Samuel Zell,
chairman of the board of trustees of Equity Office, and his affiliates.

CORNERSTONE PROPERTIES INC.
126 East 56th Street
Tower 56
New York, New York 10022
(212) 605-7100

     Cornerstone is a self-administered equity REIT which owns, through
subsidiaries, interests in 82 class A office buildings comprising approximately
18 million rentable square feet, a shopping center, a hotel and developable
land. The Cornerstone properties are primarily located in nine major
metropolitan areas throughout the United States: Atlanta, Boston, suburban
Chicago, Minneapolis, New York City, San Francisco Bay Area, Seattle, Southern
California and Washington, D.C. and surrounding suburbs. Class A office
properties are generally considered to be those that have the most favorable
locations and physical attributes, command premium rents and experience the
highest tenant retention rates within their markets.

     In January 1998, Cornerstone converted its corporate structure into an
umbrella limited partnership REIT. Under this structure, Cornerstone owns all of
its properties and conducts all of its business through Cornerstone Properties
Limited Partnership. Cornerstone is the sole general partner of, and owns the
controlling interest in, Cornerstone Partnership.

THE EQUITY OFFICE SPECIAL MEETING; VOTE REQUIRED (SEE PAGE 19)

     The special meeting of Equity Office shareholders will be held at
          on           , at   :00 a.m., local time. At the special meeting,
holders of Equity Office common shares will be asked to vote to approve the
merger agreement and the merger of Cornerstone into Equity Office. Adoption and
approval of the merger agreement and the merger require the affirmative vote of
the holders of at least a majority of the outstanding Equity Office common
shares entitled to vote at the Equity Office special meeting. The vote of the
holders of Equity Office's preferred shares is not required for approval of the
merger agreement and the merger.

     You can vote at the special meeting of Equity Office common shareholders if
you owned Equity Office common shares at the close of business on           ,
2000.

EQUITY OFFICE VOTING AGREEMENTS (SEE PAGE 72)

     Each of Equity Office's trustees and executive officers, including their
affiliates, have entered into voting agreements with Cornerstone under which
they have agreed to vote all of their Equity Office
                                        1
<PAGE>   14

shares in favor of the merger agreement and the merger, and against any
alternative acquisition proposals. As of the record date for the Equity Office
special meeting, the trustees and executive officers of Equity Office, including
their affiliates, beneficially owned, excluding share options and EOP
Partnership units held by them,           Equity Office common shares,
representing approximately      % of the outstanding Equity Office common shares
entitled to be voted at the Equity Office special meeting. Accordingly, of the
          remaining outstanding Equity Office common shares held by
nonaffiliates, only      %, or           of such shares, must be voted in favor
of the merger agreement and the merger to satisfy the vote requirement for
approval of the merger agreement and the merger by Equity Office common
shareholders.

THE CORNERSTONE SPECIAL MEETING; VOTE REQUIRED (SEE PAGE 22)

     The special meeting of Cornerstone stockholders will be held at
          on           , at   :00 a.m., local time. At the special meeting,
holders of Cornerstone common stock and holders of Cornerstone preferred stock
will be asked to consider and vote upon a proposal to approve the merger
agreement. Approval of the merger agreement requires the affirmative vote of at
least a majority of the outstanding shares of Cornerstone common stock and at
least two thirds of the outstanding shares of Cornerstone 7% cumulative
convertible preferred stock.

     You can vote at the special meeting of Cornerstone stockholders if you
owned Cornerstone common stock or Cornerstone preferred stock at the close of
business on           , 2000.

CORNERSTONE VOTING AGREEMENTS (SEE PAGE 72)

     Each of Cornerstone's directors and executive officers and Stichting
Pensioenfonds voor de Gezondheid, Geestelijke en Maatschapelijke Belangen, or
PGGM, Cornerstone's largest common stockholder, have entered into voting
agreements under which they have agreed to vote all of their shares of
Cornerstone common stock to approve the merger agreement and against any
alternative acquisition proposals. As of the record date for the Cornerstone
special meeting, the directors and executive officers of Cornerstone, including
their affiliates, and PGGM beneficially owned, excluding stock options and
Cornerstone Partnership units held by them,           shares of Cornerstone
common stock, representing approximately      % of the outstanding shares of
Cornerstone common stock entitled to be voted at the Cornerstone special
meeting. Accordingly, of the           remaining outstanding shares of
Cornerstone common stock held by nonaffiliates, only      %, or           of
such shares, must be voted to approve the merger agreement to satisfy the vote
requirement for approval of the merger agreement by Cornerstone common
stockholders.

RECOMMENDATION OF EQUITY OFFICE BOARD
(SEE PAGE 29)

     The Equity Office board of trustees has determined that the merger is in
the best interests of Equity Office and its shareholders, and recommends that
Equity Office common shareholders vote to approve the merger agreement and the
merger. Equity Office common shareholders should also refer to the reasons that
the Equity Office board considered in determining whether to approve the merger
agreement and the merger on pages 29 through 30.

RECOMMENDATION OF CORNERSTONE BOARD
(SEE PAGE 35)

     The Cornerstone board of directors has approved and adopted the merger
agreement and recommends that Cornerstone stockholders vote FOR the approval of
the merger agreement. Cornerstone stockholders should also refer to the reasons
that the Cornerstone board considered in determining whether to approve and
adopt the merger agreement on pages 35 through 37.

FAIRNESS OPINIONS OF FINANCIAL ADVISORS

EQUITY OFFICE (SEE PAGE 30)

     In deciding to adopt the merger agreement, the Equity Office board
considered the oral opinion, delivered February 10, 2000, of its financial
advisor, J.P. Morgan Securities Inc., that, as of that date, the exchange ratio
and the consideration to be paid by Equity Office to holders of Cornerstone
common stock and preferred stock in the merger, taken as a whole, was fair, from
a financial point of view, to Equity Office. Such opinion was confirmed in
writing on February 11, 2000. This opinion is attached as Annex B to this joint
proxy statement/prospectus. WE ENCOURAGE EQUITY OFFICE SHAREHOLDERS TO READ THIS
OPINION CAREFULLY.

                                        2
<PAGE>   15

CORNERSTONE (SEE PAGE 37)

     In deciding to approve and adopt the merger agreement, the Cornerstone
Board considered the oral opinion, delivered February 10, 2000, of its financial
advisor, Lazard Freres & Co. LLC, that, as of that date, the consideration to be
received in the merger was fair, from a financial point of view, to the holders
of common stock of Cornerstone. Such opinion was confirmed in writing on
February 11, 2000. This opinion is attached as Annex C to this document. WE
ENCOURAGE CORNERSTONE STOCKHOLDERS TO READ THIS OPINION CAREFULLY.

MERGER FINANCING (SEE PAGE 45)

     Equity Office intends to finance the $1.1 billion cash portion of the
purchase price to Cornerstone common stockholders by obtaining lines of credit,
issuing unsecured notes and converting its existing line of credit to a term
loan. At           , 2000, Equity Office had unused borrowing capacity under its
existing line of credit of approximately $776.5 million.

AMENDMENTS TO EQUITY OFFICE'S DECLARATION OF TRUST (SEE PAGE 53)

     As part of the merger, the Equity Office declaration of trust will be
amended to add a limitation on the ownership by non-U.S. persons of Equity
Office common shares and preferred shares issued in the future.

THE MERGER AGREEMENT (SEE PAGE 55)

     The merger agreement is attached at the back of this joint proxy
statement/prospectus as Annex A. We urge you to read the merger agreement as it
is the legal document that governs the merger.

     The merger agreement contemplates the following two-step transaction:

     - the partnership merger, whereby Cornerstone Partnership will merge into
       EOP Partnership and Cornerstone Partnership will cease to exist; followed
       by

     - the merger, whereby Cornerstone will merge into Equity Office and
       Cornerstone will cease to exist.

     - The merger will be completed immediately after the completion of the
       partnership merger.

     The diagram of the structure of the mergers is as follows:

     [DIAGRAM OF STRUCTURE OF THE PARTIES BEFORE THE MERGERS AND AFTER THE
MERGERS]

CONDITIONS TO THE PARTNERSHIP MERGER AND THE MERGER (SEE PAGE 62)

     Before the partnership merger and the merger can be completed, a number of
conditions must be satisfied. These include:

     - approval of the merger agreement and the merger by Equity Office common
       shareholders and approval of the merger agreement by the Cornerstone
       common and preferred stockholders;

     - approval of the merger agreement and the partnership merger by the
       holders of limited partner interests in Cornerstone Partnership and the
       holders of limited partner interests in EOP Partnership;

     - absence of any court order or law preventing the consummation of either
       the merger or the partnership merger;

     - delivery of tax opinions that the merger qualifies as a reorganization
       under section 368(a) of the Internal Revenue Code; and

     - other customary closing conditions.

     Where the law permits, Equity Office or Cornerstone could decide to
complete the merger even though one or more conditions were not satisfied. By
law, neither Equity Office nor Cornerstone can waive (A) the requirement that
shareholders adopt and approve the merger agreement and the merger, (B) a
failure by the limited partners of EOP Partnership or Cornerstone Partnership to
approve the merger agreement and the partnership merger or (C) any court order
or law preventing consummation of the merger. Whether any of the conditions
would be waived would depend on the facts and circumstances as determined by the
reasonable business judgment of the board of trustees of Equity Office or the
board of directors of Cornerstone. If Equity Office or Cornerstone waived
compliance with one or more of the conditions and the condition was deemed
material to a vote of Equity Office shareholders and/or Cornerstone
stockholders, Equity Office and/or Cornerstone would have to resolicit the

                                        3
<PAGE>   16

approval by its stockholders before the closing of the merger.

     It is a condition to the consummation of the merger that Hogan & Hartson
L.L.P., counsel to Equity Office, and King & Spalding, counsel to Cornerstone,
deliver opinions that the merger qualifies as a reorganization under the
provisions of section 368(a) of the Code. This condition will not be waived.

TERMINATION OF THE MERGER AGREEMENT;
TERMINATION FEES (SEE PAGE 66)

     The merger agreement contains provisions addressing the circumstances under
which Equity Office or Cornerstone may terminate the merger agreement. In
addition, the merger agreement provides that, in several circumstances,
Cornerstone may be required to pay to Equity Office a termination fee of $100
million.

ACCOUNTING TREATMENT (SEE PAGE 53)

     The merger will be treated as a purchase for financial accounting purposes.

CONFLICTS OF INTEREST OF CORNERSTONE DIRECTORS AND OFFICERS IN THE MERGER (SEE
PAGE 42)

     A number of directors and officers of Cornerstone have interests in the
merger as employees and/or directors that are different from, or in addition to,
yours as a Cornerstone stockholder. The Cornerstone board recognized these
interests and determined that they did not affect the benefits of the merger to
Cornerstone stockholders.

     If the merger takes place, William Wilson III, John S. Moody and Jan H.W.R.
van der Vlist will become members of the Equity Office board of trustees. Also,
indemnification arrangements in the merger agreement and directors' and
officers' liability insurance for existing directors and officers of Cornerstone
will be continued by Equity Office after the merger.

     In addition, if the merger takes place, options to purchase Cornerstone
common stock held by Cornerstone's directors and officers will be automatically
converted into options to acquire Equity Office common shares adjusted to
account for the exchange ratio of 0.7009. Further, if the merger is completed
and Cornerstone executives with whom Cornerstone has severance agreements are
terminated under the circumstances described in "The Merger -- Interests of
Cornerstone's Directors, Officers and Significant Stockholders in the Merger" on
page 42, then such persons would be entitled to aggregate benefits, if the
employment of all of these persons is terminated following completion of the
merger, of approximately $34.7 million under such agreements. Upon the
completion of the merger, outstanding unvested options to purchase an aggregate
of 3,675,647 shares of Cornerstone common stock will become exercisable in full.
Of these, Messrs. Wilson, Moody and the other Cornerstone executives with
severance agreements held unvested options to purchase 3,177,476 shares of
Cornerstone common stock at a per share weighted average exercise price of
$16.48.

TRUSTEES OF EQUITY OFFICE FOLLOWING THE MERGER (SEE PAGE 41)

     The merger agreement provides that Messrs. Moody, Wilson and van der Vlist
will become members of the board of trustees of Equity Office, with terms
expiring in 2002, 2003 and 2003, respectively. In addition, Equity Office has
agreed to renominate Mr. van der Vlist, or a replacement designated by PGGM and
subject to the approval of Equity Office if such replacement is not an officer,
director or employee of PGGM, to the board of trustees of Equity Office upon the
expiration of his terms in 2003 and 2006, so long as PGGM and its affiliates
continue to own in the aggregate 21,000,000 or more of the outstanding Equity
Office common shares.

DIFFERENCES IN THE RIGHTS OF SHAREHOLDERS
(SEE PAGE 74)

     The rights of holders of Cornerstone common stock are currently governed by
Nevada law and Cornerstone's articles of incorporation and bylaws. Upon
completion of the merger, the rights of former holders of Cornerstone common
stock who receive Equity Office common shares in the merger will be governed by
Maryland law and Equity Office's declaration of trust and bylaws.

                                        4
<PAGE>   17

                SELECTED HISTORICAL CONSOLIDATED FINANCIAL DATA

EQUITY OFFICE

     The following table sets forth selected consolidated and combined financial
and operating information on a historical basis for Equity Office and its
predecessors. The selected financial data has been derived from the historical
consolidated or combined financial statements of Equity Office and its
predecessors, audited by Ernst & Young LLP, independent auditors. The following
information should be read in conjunction with the consolidated and combined
financial statements and financial statement notes of Equity Office and its
predecessors incorporated by reference in this joint proxy statement/prospectus.
See "Where You Can Find More Information" on page 90.

<TABLE>
<CAPTION>
                                   EQUITY OFFICE PROPERTIES TRUST                 EQUITY OFFICE PREDECESSORS
                             -------------------------------------------   -----------------------------------------
                                                              FOR THE          FOR THE
                                                              PERIOD           PERIOD
                                    FOR THE YEARS              FROM             FROM              FOR THE YEARS
                                        ENDED              JULY 11, 1997   JANUARY 1, 1997            ENDED
                                    DECEMBER 31,              THROUGH          THROUGH            DECEMBER 31,
                             ---------------------------   DECEMBER 31,       JULY 10,       -----------------------
                                 1999           1998           1997             1997            1996         1995
                             ------------   ------------   -------------   ---------------   ----------   ----------
                                                  (DOLLARS IN THOUSANDS, EXCEPT PER SHARE DATA)
<S>                          <C>            <C>            <C>             <C>               <C>          <C>
OPERATING DATA:
Revenues:
  Rental, parking and
    other..................  $  1,919,056   $  1,658,420   $    406,713       $ 327,017      $  493,396   $  356,959
                             ------------   ------------   ------------       ---------      ----------   ----------
         Total revenues....     1,942,243      1,679,699        412,968         339,104         508,124      371,457
                             ------------   ------------   ------------       ---------      ----------   ----------
Expenses:
  Interest.................       413,995        338,611         76,675          80,481         119,595      100,566
  Depreciation and
    amortization...........       358,989        305,982         70,346          66,034          96,237       74,156
  Property operating and
    ground rent(1).........       669,763        600,367        155,679         127,285         201,067      151,488
  General and
    administrative.........        80,927         63,564         17,690          17,201          23,145       21,987
  Provision for value
    impairment.............            --             --             --              --              --       20,248
                             ------------   ------------   ------------       ---------      ----------   ----------
         Total expenses....     1,523,674      1,308,524        320,390         291,001         440,044      368,445
                             ------------   ------------   ------------       ---------      ----------   ----------
Income before allocation to
  minority interests,
  income from investment in
  unconsolidated joint
  ventures, net gain/(loss)
  on sales of real estate
  and extraordinary
  items....................       418,569        371,175         92,578          48,103          68,080        3,012
Minority interests.........       (50,153)       (38,340)        (7,799)           (912)         (2,086)      (2,129)
Income from investment in
  unconsolidated joint
  ventures.................        13,824         11,267          3,173           1,982           2,093        2,305
Net gain/(loss) on sales of
  real estate and
  extraordinary items......        49,113          4,927        (16,240)         12,236           5,338       31,271
                             ------------   ------------   ------------       ---------      ----------   ----------
Net income.................       431,353        349,029         71,712          61,409          73,425       34,459
Put option settlement......        (5,658)            --             --              --              --           --
Preferred distributions....       (43,603)       (32,202)          (649)             --              --           --
                             ------------   ------------   ------------       ---------      ----------   ----------
Net income available for
  common shares............  $    382,092   $    316,827   $     71,063       $  61,409      $   73,425   $   34,459
                             ============   ============   ============       =========      ==========   ==========
Net income available per
  weighted average common
  share
  outstanding -- basic.....  $       1.49   $       1.25   $       0.44
                             ============   ============   ============
</TABLE>

                                        5
<PAGE>   18

<TABLE>
<CAPTION>
                                   EQUITY OFFICE PROPERTIES TRUST                 EQUITY OFFICE PREDECESSORS
                             -------------------------------------------   -----------------------------------------
                                                              FOR THE          FOR THE
                                                              PERIOD           PERIOD
                                    FOR THE YEARS              FROM             FROM              FOR THE YEARS
                                        ENDED              JULY 11, 1997   JANUARY 1, 1997            ENDED
                                    DECEMBER 31,              THROUGH          THROUGH            DECEMBER 31,
                             ---------------------------   DECEMBER 31,       JULY 10,       -----------------------
                                 1999           1998           1997             1997            1996         1995
                             ------------   ------------   -------------   ---------------   ----------   ----------
                                                  (DOLLARS IN THOUSANDS, EXCEPT PER SHARE DATA)
<S>                          <C>            <C>            <C>             <C>               <C>          <C>
Net income available per
  weighted average common
  share
  outstanding -- diluted...  $       1.48   $       1.24   $       0.43
                             ============   ============   ============
Weighted average common
  shares
  outstanding -- basic.....   256,045,895    253,167,037    162,591,477
                             ============   ============   ============
Weighted average common
  shares
  outstanding -- diluted...   291,157,204    283,974,532    180,014,027
                             ============   ============   ============
BALANCE SHEET DATA (at end
  of period):
  Investment in real
    estate, net of
    accumulated
    depreciation...........  $ 12,572,153   $ 13,331,560   $ 10,976,319              --      $3,291,815   $2,393,403
  Total assets.............  $ 14,046,058   $ 14,261,291   $ 11,751,672              --      $3,912,565   $2,650,890
  Mortgage debt, unsecured
    notes and lines of
    credit.................  $  5,851,918   $  6,025,405   $  4,284,317              --      $1,964,892   $1,434,827
  Total liabilities........  $  6,336,531   $  6,472,613   $  4,591,697              --      $2,174,483   $1,529,334
  Minority interests.......  $    883,454   $    737,715   $    754,818              --      $   11,080   $   31,587
  Preferred shares.........  $    615,000   $    615,000   $    200,000              --              --           --
  Shareholders' equity/
    Owners' equity.........  $  6,211,073   $  6,435,963   $  6,205,157              --      $1,727,002   $1,089,969
OTHER DATA:
  General and
    administrative expenses
    as a percentage of
    total revenues.........           4.2%           3.8%           4.3%            5.1%            4.6%         5.9%
  Number of office
    properties.............           294            284            258              --              84           73
  Net rentable square feet
    of office properties
    (in millions)..........          77.0           75.1           65.3              --            29.2         23.1
  Occupancy of office
    properties.............            94%            95%            94%             --              90%          86%
  Number of parking
    facilities.............            20             19             17              --              10            3
  Number of spaces at
    parking facilities.....        20,506         18,059         16,749              --           7,321        3,323
  Funds from
    Operations(2)..........  $    748,983   $    662,585   $    163,253       $ 113,022      $  160,460   $   96,104
                             ============   ============   ============       =========      ==========   ==========
  Property net operating
    income(3)..............  $  1,256,180   $  1,065,714   $    253,418       $ 202,108      $  294,556   $  206,341
                             ============   ============   ============       =========      ==========   ==========
  Earnings before interest,
    taxes, depreciation,
    and amortization(4)....  $  1,228,913   $  1,049,577   $    242,969       $ 197,489      $  286,128   $  200,438
                             ============   ============   ============       =========      ==========   ==========
  Cash flow provided by
    operating activities...  $    720,711   $    759,151   $    190,754       $  95,960      $  165,975   $   93,878
                             ============   ============   ============       =========      ==========   ==========
  Cash flow (used for)
    investing activities...  $    (67,138)  $ (2,231,712)  $ (1,592,272)      $(571,068)     $ (924,227)  $ (380,615)
                             ============   ============   ============       =========      ==========   ==========
  Cash flow (used for)
    provided by financing
    activities.............  $   (718,315)  $  1,310,788   $  1,630,346       $ 245,851      $1,057,551   $  276,513
                             ============   ============   ============       =========      ==========   ==========
</TABLE>

                                        6
<PAGE>   19

- ---------------
(1) Property operating expenses includes real estate taxes, insurance, repairs
    and maintenance and other property operating expenses.

(2) The White Paper on Funds from Operations approved by the Board of Governors
    of the National Association of Real Estate Investment Trusts ("NAREIT") in
    March 1995 defines Funds from Operations as net income (loss), computed in
    accordance with GAAP, excluding gains (or losses) from debt restructuring
    and sales of properties, plus real estate related depreciation and
    amortization and after adjustments for unconsolidated partnerships and joint
    ventures. Equity Office believes that Funds from Operations is helpful to
    investors as a measure of the performance of an equity REIT because, along
    with cash flow from operating, investing and financing activities, it
    provides investors with an indication of the ability of Equity Office to
    incur and service debt, to make capital expenditures and to fund other cash
    needs. Equity Office computes Funds from Operations in accordance with
    standards established by NAREIT, which may not be comparable to Funds from
    Operations reported by other REITs that do not define the term in accordance
    with the current NAREIT definition or that interpret the current NAREIT
    definition differently than Equity Office. Funds from Operations does not
    represent cash generated from operating activities in accordance with GAAP,
    nor does it represent cash available to pay distributions and should not be
    considered as an alternative to net income, determined in accordance with
    GAAP, as an indication of Equity Office's financial performance or to cash
    flow from operating activities, determined in accordance with GAAP, as a
    measure of Equity Office's liquidity, nor is it indicative of funds
    available to fund Equity Office's cash needs, including its ability to make
    cash distributions.

(3) Property net operating income is defined as rental income including tenant
    reimbursements, parking and other income less property operating expenses
    including real estate taxes, insurance, repairs and maintenance and other
    property operating expenses.

(4) Earnings before interest, taxes, depreciation and amortization is defined as
    net income excluding interest expense, taxes, depreciation and amortization,
    minority interest allocation to EOP Partnership, net gain/(loss) on sales of
    real estate, gains/losses from extraordinary items and income from
    investment in unconsolidated joint ventures plus Equity Office's share of
    the earnings before interest, taxes, depreciation and amortization for the
    unconsolidated joint ventures. Earnings before interest, taxes, depreciation
    and amortization is presented because Equity Office believes this data is
    used by some investors to evaluate Equity Office's ability to meet debt
    service requirements. Equity Office considers earnings before interest,
    taxes, depreciation and amortization to be an indicative measure of its
    operating performance due to the significance of Equity Office's long-lived
    assets and because this data can be used to measure Equity Office's ability
    to service debt, fund capital expenditures and expand its business. However,
    this data should not be considered as an alternative to net income,
    operating profit, cash flows from operations or any other operating or
    liquidity performance measure prescribed by GAAP. In addition, earnings
    before interest, taxes, depreciation and amortization as calculated by
    Equity Office may not be comparable to similarly titled measures reported by
    other companies. Interest expense, taxes, depreciation and amortization,
    which are not reflected in the presentation of earnings before interest,
    taxes, depreciation and amortization, have been, and will be, incurred by
    Equity Office. Investors are cautioned that these excluded items are
    significant components in understanding and assessing Equity Office's
    financial performance.

                                        7
<PAGE>   20

CORNERSTONE

     The following table sets forth selected consolidated financial and
operating information on a historical basis for Cornerstone. The selected
financial data has been derived from the historical consolidated financial
statements of Cornerstone audited by PricewaterhouseCoopers LLP, independent
accountants. The following information should be read in conjunction with the
consolidated financial statements and financial statement notes of Cornerstone
incorporated by reference in this joint proxy statement/prospectus. See "Where
You Can Find More Information" on page 90.

<TABLE>
<CAPTION>
                                    1999          1998          1997         1996        1995
                                 ----------    ----------    ----------    --------    ---------
                                            (IN THOUSANDS, EXCEPT PER SHARE AMOUNTS)
<S>                              <C>           <C>           <C>           <C>         <C>
BALANCE SHEET DATA:
Real estate investments before
  accumulated depreciation.....  $3,951,465    $4,262,136    $1,947,272    $799,662    $ 706,988
Total assets...................   4,170,228     4,281,984     2,051,481     766,180      586,089
Long-term debt.................   1,448,331     1,532,474       706,178     400,142      369,600
Credit facility................     329,000       465,000       187,000          --           --
Total liabilities..............   1,890,636     2,138,309       940,062     442,375      403,927
Redeemable preferred stock.....  $       --    $       --    $       --    $162,743    $      --
OPERATING DATA:
Revenues.......................  $  617,448    $  359,486    $  173,911    $116,908    $  92,387
Expenses.......................     465,675       262,173       134,041     106,646       90,427
Gain (loss) on sale of real
  estate assets................     131,034        (2,076)           --          --           --
Minority interest..............      40,737         7,469         2,368       1,519        3,417
Gain (loss) on interest rate
  swap.........................          --            --            99       4,278       (7,672)
Cumulative effect of a change
  in accounting principle......        (630)           --            --          --           --
Extraordinary loss.............      10,787         4,303            54       3,925        4,445
Net income (loss)..............  $  230,653    $   83,465    $   37,547    $  9,096    $ (13,574)
Net income per common share,
  Basic........................  $     1.76    $     0.80    $     0.63    $   0.19    $   (0.94)
Net income per common share,
  Diluted......................  $     1.74    $     0.80    $     0.63    $   0.19    $   (0.94)
Dividends declared per common
  share........................  $     0.90    $     1.50    $     1.04    $   1.20    $    1.16
OTHER DATA:
Cash Flow from:
  Operations...................  $  223,505    $  182,797    $   65,922    $ 34,522    $  20,036
  Investing....................      52,488      (823,082)     (462,837)    (57,259)    (135,527)
  Financing....................    (318,183)      677,424       306,842     129,800      110,725
  Funds From Operations(1).....  $  242,426    $  155,915    $   68,308    $ 34,719    $  21,424
</TABLE>

- ---------------
(1) Funds from Operations is a calculation which is defined by NAREIT and is not
    indicative of either net income or cash flow from operations as calculated
    in accordance with GAAP as a measure of performance or liquidity.

EQUIVALENT PER SHARE DATA

     We have summarized below the per share information for our respective
companies on an historical basis, pro forma combined basis and pro forma
combined equivalent basis. The pro forma combined summary amounts are based on
the purchase method of accounting. The Cornerstone per share pro forma combined
equivalents are calculated by multiplying the pro forma combined per share
amounts by the

                                        8
<PAGE>   21

exchange ratio of 0.7009. Cornerstone stockholders who receive Equity Office
common shares in the merger will receive 0.7009 of an Equity Office common share
in exchange for each share of Cornerstone common stock. You should note that in
1999, Cornerstone declared distributions equal to $.90 per share of Cornerstone
common stock, but paid distributions equal to $1.20 per share of Cornerstone
common stock during such period.

     The following information should be read together with the historical and
pro forma financial statements included or incorporated by reference in this
document. See "Where You Can Find More Information" on page 90.

<TABLE>
<CAPTION>
                                                              FOR THE YEAR ENDED
                                                              DECEMBER 31, 1999
                                                              ------------------
                                                              BASIC     DILUTED
                                                              ------    --------
<S>                                                           <C>       <C>
NET INCOME PER COMMON SHARE(1):
  Equity Office.............................................  $1.49      $1.48
  Cornerstone...............................................  $1.76      $1.74
  Equity Office and Cornerstone pro forma combined before
     extraordinary items and cumulative effect of a change
     in accounting principle................................  $1.81      $1.81
  Cornerstone pro forma combined equivalent.................  $1.27      $1.27
</TABLE>

- ---------------
(1) The net income per common share amounts include the gain on sales of real
    estate in 1999 for Equity Office and Cornerstone. Such gains can fluctuate
    substantially from year to year. Therefore, the following table reflects net
    income per share before extraordinary items, cumulative effect of a change
    in accounting principle and the gains on sales of real estate for Equity
    Office and Cornerstone:

<TABLE>
<CAPTION>
                                                              BASIC    DILUTED
                                                              -----    -------
<S>                                                           <C>      <C>
NET INCOME BEFORE EXTRAORDINARY ITEMS, CUMULATIVE EFFECT OF
  A CHANGE IN ACCOUNTING PRINCIPLE AND GAIN ON SALES OF REAL
  ESTATE PER COMMON SHARE:
  Equity Office.............................................  $1.32     $1.31
  Cornerstone...............................................  $0.96     $0.96
  Equity Office and Cornerstone pro forma combined..........  $1.28     $1.28
  Cornerstone pro forma combined equivalent.................  $0.89     $0.89
</TABLE>

<TABLE>
<CAPTION>
                                                                FOR THE YEAR ENDED
                                                                DECEMBER 31, 1999
                                                                ------------------
<S>                                                             <C>
CASH DISTRIBUTIONS DECLARED PER COMMON SHARE:
  Equity Office.............................................          $ 1.58
  Cornerstone...............................................          $ 0.90
  Equity Office and Cornerstone pro forma combined..........          $ 1.58
  Cornerstone pro forma combined equivalent.................          $ 1.11
SHAREHOLDERS' EQUITY (BOOK VALUE) PER COMMON SHARE (END OF
  PERIOD):
  Equity Office.............................................          $24.69
  Cornerstone...............................................          $14.87
  Equity Office and Cornerstone pro forma combined..........          $24.87
  Cornerstone pro forma combined equivalent.................          $17.43
</TABLE>

                                        9
<PAGE>   22

MARKET PRICES OF COMMON SHARES

     The following table sets forth the price per share of Equity Office common
shares and Cornerstone common stock based on the last reported sale prices per
share on the NYSE on February 10, 2000, the last trading day prior to the public
announcement of the execution of the merger agreement, and on                ,
2000, the most recent date for which prices are available prior to mailing this
joint proxy statement/prospectus.

<TABLE>
<CAPTION>
                                                                       PRICE PER SHARE
                                                                                       CORNERSTONE PRO
                                                     EQUITY OFFICE    CORNERSTONE    FORMA EQUIVALENT(1)
                                                     -------------    -----------    -------------------
<S>                                                  <C>              <C>            <C>
February 10, 2000..................................    $25.6875         $14.875            $18.00
               , 2000(2)...........................    $                $                  $
</TABLE>

- ---------------
(1) Computed by multiplying the average Equity Office common share closing price
    for the ten business days ended February 4, 2000 by the 0.7009 exchange
    ratio.

(2) The average last reported sale price per Equity Office common share for the
    five trading days preceding                , 2000 was $               . The
    Cornerstone pro forma equivalent, based on such five trading days, would be
    $        .

                                       10
<PAGE>   23

         SUMMARY UNAUDITED PRO FORMA CONDENSED COMBINED FINANCIAL DATA

     The following table sets forth the summary unaudited pro forma combined
financial data for Equity Office and Cornerstone as a combined entity, giving
effect to the merger as if it had occurred on the dates indicated herein and
after giving effect to the pro forma adjustments. The unaudited pro forma
combined operating data are presented as if the merger had been consummated on
January 1, 1999. The unaudited pro forma combined balance sheet data at December
31, 1999 is presented as if the merger had occurred on December 31, 1999. In the
opinion of management of Equity Office, all adjustments necessary to reflect the
effects of these transactions have been made. The merger will be accounted for
under the purchase method of accounting in accordance with Accounting Principles
Board Opinion No. 16.

     The pro forma financial information should be read in conjunction with, and
is qualified in its entirety by, the respective historical audited financial
statements and financial statement notes of Equity Office and of Cornerstone
incorporated by reference into this joint proxy statement/prospectus. The
unaudited pro forma operating data are presented for comparative purposes only
and are not necessarily indicative of what the actual combined results of
operations of Equity Office and Cornerstone would have been for the period
presented, nor does such data purport to represent the results of future
periods. See "Equity Office Properties Trust Pro Forma Condensed Combined
Financial Statements" on page F-1.

<TABLE>
<CAPTION>
                                                                          PRO FORMA
                                                                     FOR THE YEAR ENDED
                                                                      DECEMBER 31, 1999
                                                              ---------------------------------
                                                                   (AMOUNTS IN THOUSANDS,
                                                                   EXCEPT PER SHARE DATA)
<S>                                                           <C>
OPERATING DATA:
Revenue:
  Rental....................................................            $  1,954,581
  Tenant reimbursements.....................................                 402,743
  Parking...................................................                 139,547
  Other.....................................................                  38,028
  Fee income................................................                  11,616
  Interest/dividends........................................                  18,459
                                                                        ------------
          Total revenues....................................               2,564,974
                                                                        ------------
Expenses:
  Interest:
     Expense incurred.......................................                 639,390
     Amortization of deferred financing costs...............                  10,366
  Depreciation..............................................                 444,032
  Amortization..............................................                  14,545
  Real estate taxes.........................................                 315,332
  Insurance.................................................                  13,137
  Repairs and maintenance...................................                 284,358
  Property operating........................................                 252,826
  Ground rent...............................................                   8,294
  General and administrative................................                  88,933
                                                                        ------------
          Total expenses....................................               2,071,213
                                                                        ------------
Income before allocation to minority interests, income from
  investment in unconsolidated joint ventures and net gain
  on sales of real estate...................................                 493,761
</TABLE>

                                       11
<PAGE>   24

<TABLE>
<CAPTION>
                                                                          PRO FORMA
                                                                     FOR THE YEAR ENDED
                                                                      DECEMBER 31, 1999
                                                              ---------------------------------
                                                                   (AMOUNTS IN THOUSANDS,
                                                                   EXCEPT PER SHARE DATA)
<S>                                                           <C>
Minority interests:
  EOP Partnership...........................................                 (87,698)
  Partially owned properties................................                  (7,736)
Income from investment in unconsolidated joint ventures.....                  14,546
Net gain on sales of real estate............................                 190,695
                                                                        ------------
Net income from continuing operations.......................                 603,568
Preferred distributions.....................................                 (43,603)
Put option settlement.......................................                  (5,658)
                                                                        ------------
Net income from continuing operations before extraordinary
  items and cumulative effect of a change in accounting
  principle available for common shares.....................            $    554,307
                                                                        ============
Net income from continuing operations before extraordinary
  items and cumulative effect of a change in accounting
  principle per weighted average common share
  outstanding -- basic......................................            $       1.81
                                                                        ============
Weighted average common shares outstanding -- basic.........             305,424,355
                                                                        ============
Net income from continuing operations before extraordinary
  items and cumulative effect of a change in accounting
  principle per weighted average common share
  outstanding -- diluted....................................            $       1.81
                                                                        ============
Weighted average common shares outstanding -- diluted.......             353,945,132
                                                                        ============
BALANCE SHEET DATA (AT END OF PERIOD):
  Investment in real estate net of accumulated
     depreciation...........................................            $ 16,905,340
                                                                        ============
  Total Assets..............................................            $ 18,715,593
                                                                        ============
  Total Debt................................................            $  8,773,144
                                                                        ============
  Total Liabilities.........................................            $  9,371,062
                                                                        ============
  Minority Interests........................................            $  1,245,604
                                                                        ============
  Shareholders' Equity......................................            $  8,098,927
                                                                        ============
</TABLE>

                                       12
<PAGE>   25

                                  RISK FACTORS

     In addition to the risks relating to the businesses of Equity Office and
Cornerstone, which are incorporated by reference in this joint proxy
statement/prospectus from other SEC filings, and the other information included
in this document, including the matters addressed in "A Warning About Forward-
Looking Statements," you should carefully consider the following risk factors
relating to the merger in determining whether or not to vote in favor of the
approval of the merger agreement and the merger.

CORNERSTONE COMMON STOCKHOLDERS MAY RECEIVE EQUITY OFFICE COMMON SHARES IN THE
MERGER WITH A VALUE OF LESS THAN $18.00 PER SHARE

     There can be no assurance that any Equity Office common shares you receive
in the merger will have a value, or trade at a price, equal to $18.00, the cash
price, on a per share equivalent basis at the time of the merger. The market
price of the Equity Office common shares at the time of the merger may vary
significantly from the expected prices on the date of execution of the merger
agreement or from the prices in effect on either the date of this joint proxy
statement/prospectus or the date of the Equity Office and Cornerstone special
meetings. These variances may arise due to, among other things, changes in the
business, operations and prospects of Equity Office, market assessments of the
likelihood that the merger will be completed, and interest rates, general market
and economic conditions and other factors. It should be noted that during the
12-month period ending on             , 2000, the most recent date practicable,
the closing per share price of Equity Office common shares varied from a low of
$     to a high of $
and ended that period at $     . Historical trading markets are not necessarily
indicative of future performance.

     The exchange ratio for shares of Cornerstone common stock to be converted
into Equity Office common shares in the merger was fixed at the time of the
signing of the merger agreement and is not subject to adjustment based on
changes in the trading price of Equity Office common shares or Cornerstone
common stock before the closing of the merger. Accordingly, the market value of
any Equity Office common shares that Cornerstone common stockholders receive in
the merger will depend on the market value of Equity Office common shares at the
time of completion of the merger.

THE DIRECTORS AND OFFICERS OF CORNERSTONE MAY HAVE INTERESTS IN THE COMPLETION
OF THE MERGER THAT ARE DIFFERENT FROM THE INTERESTS OF CORNERSTONE'S
STOCKHOLDERS

     In considering the recommendation of the Cornerstone board with respect to
the merger, Cornerstone stockholders should be aware that, as described below,
the Cornerstone directors and officers described below and PGGM have interests
in the merger that differ from, or are in addition to, the interests of
stockholders generally.

     Trustees of Equity Office After the Merger.  Following the merger, the
current trustees of Equity Office will remain as trustees of the combined
entity. In addition, three current directors of Cornerstone, John S. Moody, Jan
H.W.R. van der Vlist and William Wilson III, will become trustees of Equity
Office, with their terms expiring in 2002, 2003 and 2003, respectively. Under
the terms of the merger agreement, Equity Office is obligated to nominate Mr.
van der Vlist for reelection to the board of trustees in 2003 and 2006 if PGGM
and its affiliates continue to own at least 21,000,000 or more of the issued and
outstanding Equity Office common shares, as adjusted for stock splits or similar
actions, at all times up to the time the trustees are being elected in those
years. If Mr. van der Vlist fails to stand for re-election in 2003 or 2006 for
any reason, or in the event of his death or earlier resignation, and if PGGM
continues to own at least 21,000,000 or more of the issued and outstanding
Equity Office shares as described above, Equity Office is required to take all
action necessary to nominate a replacement designated by PGGM for election or
re-election as a trustee for an additional three-year term, subject to Equity
Office's approval if the replacement is not an officer, director or employee of
PGGM.

     Equity-Based Awards.  Under Cornerstone's incentive stock plans, an
aggregate of 89,113 unvested shares of restricted common stock previously
awarded to the executives who are parties to the retention agreements described
below will vest in full upon the merger and will be converted into cash or
common
                                       13
<PAGE>   26

shares of Equity Office according to the terms of the merger agreement. In
addition, unvested options to purchase an aggregate of 3,177,476 shares of
Cornerstone common stock previously awarded to such executives will vest in full
upon the merger, and each option may be surrendered by the holder of the option
to Equity Office for cash equal to the difference between $18.00 per share and
the exercise price of the option or exchanged for options to purchase common
shares of Equity Office according to the terms of the merger agreement.
Cornerstone and Equity Office have agreed that the merger will constitute a
change in control for purposes of all applicable Cornerstone benefit and
compensation plans, including the Cornerstone 1998 Long Term Incentive Plan and
all stock option and restricted stock agreements provided under the terms of
such plans.

     Mr. Moody has agreed with Cornerstone to waive the acceleration of his
unvested restricted stock and unvested stock options.

     Moody Agreement.  Cornerstone has entered into a contract with Mr. Moody
whereby amounts are accrued under an unfunded arrangement to pay Mr. Moody a
supplemental pension. Under the contract, his supplemental pension account was
established with a credit of $250,000 as of July 1, 1995, and Cornerstone is
obligated to credit the account in the amount of $60,000 each subsequent July 1
during the continuance of Mr. Moody's employment.

     The account is also credited with any deemed income, gains or losses which
would be attributable to a corresponding investment of an equal cash amount in
such investment as Cornerstone, taking into account Mr. Moody's views, shall
deem the account to be invested. Payment of Mr. Moody's supplemental pension
account is to be made in a lump sum as of the date Mr. Moody terminates his
employment. However, if his employment is terminated for cause, Mr. Moody will
forfeit the account and no payment will be made. The contract also provides
that, outside of a change in control context, if Mr. Moody's employment
terminates for a reason other than cause, an additional credit of $60,000 (or
less, if Mr. Moody resigns without good reason) will be made to the account.
Following a change in control, in the event Mr. Moody's employment is terminated
by Cornerstone other than for cause, or if he resigns for good reason,
Cornerstone is obligated to credit his supplemental pension account with an
amount equal to $60,000 times the number of years, including fractional portions
of a year, remaining between his age on the date his employment ceases and age
60.

     Retention Agreements.  Cornerstone has entered into retention agreements
with some of its officers to address the terms and conditions of their
employment in the event of a change in control. A "change in control" is
generally said to occur when: (a) any person becomes the owner of 25% or more of
Cornerstone's voting securities; (b) directors who constitute the Cornerstone
board at the beginning of any two-year period, and any new directors whose
election or nomination for election was approved by a vote of at least three
quarters of the directors then in office who either were directors at the
beginning of such period or whose election or nomination for election was
previously so approved, cease to constitute a majority; (c) a merger,
consolidation or reorganization in which Cornerstone's voting securities do not
continue to represent at least 50% of the surviving entity; or (d) a
reorganization, liquidation, or sale of all or substantially all of
Cornerstone's assets.

     In the event of termination of employment by Cornerstone without cause, or
by the employee for good reason, within two years following a change in control,
Cornerstone will make a lump sum payment of: (a) the pro rata portion of the
annual bonus for the year termination occurred, calculated on the basis of the
target bonus and on the assumption that all performance goals have been or will
be achieved; and (b) three times annual compensation in the case of Messrs.
Wilson and Moody and H. Lee Van Boven and Rodney C. Dimock, and two times annual
compensation in the case of all other officers. Annual compensation is generally
defined as the salary in effect immediately before termination or change in
control, plus the highest of the target annual bonus for the termination year or
the average of the three annual bonuses paid immediately prior to the
termination or the change in control.

     An officer may be terminated for "cause" if he or she is convicted of a
felony, reveals confidential information about Cornerstone or refuses to
substantially perform his duties. An officer can terminate for "good reason" if
Cornerstone reduces his salary or bonus target, reduces the value of employee
benefits
                                       14
<PAGE>   27

materially, relocates his office more then 25 miles from the present location,
detrimentally alters his title or position, fails to get a successor to agree to
assume the obligations under the retention agreement or materially breaches any
provision of the agreement.

     In addition, the officer and his eligible dependents will continue to be
eligible to participate during the benefit continuation period in the medical,
dental, health, life and other welfare benefit plans and arrangements applicable
to him immediately before his or her termination of employment, on the same
terms and conditions in effect immediately before such termination. "Benefit
continuation period" means the period beginning on the day of termination and
ending on the earlier to occur of: (a) the third anniversary of the date of
termination in the case of Messrs. Wilson, Moody, Van Boven and Dimock and the
second anniversary of the date of termination in the case of all other officers;
and (b) the date that the officer is eligible and elects coverage under the
plans of a subsequent employer which provide substantially equivalent or greater
benefits to the officer and his dependents.

     If the payments to an officer under the retention agreement constitute
"parachute payments," as defined in Section 280G of the Internal Revenue Code,
and the officer would receive more value after taxes if the payments to be made
to the officer were capped at three times the "base amount" less $1.00, then the
payments will be reduced to that amount.

     Equity Office has agreed to assume and to perform Cornerstone's obligations
under the retention agreements in the same manner and to the same extent that
Cornerstone would be required to perform such obligations if the merger had not
taken place. The parties have agreed that the merger will be a "change in
control" for purposes of the retention agreements.

     Mr. Moody has agreed to waive any rights to payment of the lump sum payment
and to the continuation of benefits under his retention agreement in
consideration of Cornerstone's payment to Mr. Moody upon completion of the
merger, in recognition of his past service to Cornerstone, of $7.3 million or,
if the merger is completed on or after July 1, 2000, $6.8 million. Messrs. Van
Boven and Dimock have each agreed to waive the payment of $1.0 million, and R.
Matthew Moran has agreed to waive the payment of $500,000, of the lump sum
payment otherwise payable to each of them in the event of the termination of
their employment other than for cause after the merger.

     Noncompetition Agreements.  Each of Messrs. Moody, Van Boven, Dimock and
Moran have agreed to enter into noncompetition agreements with Equity Office
that will restrict their ability to engage in business activities that are
competitive with those of Equity Office after the merger. In order to induce
Messrs. Moody, Van Boven, Dimock and Moran to enter into such agreements, Equity
Office will pay $5.0 million to Mr. Moody, $1.0 million to each of Messrs. Van
Boven and Dimock and $500,000 to Mr. Moran upon the completion of the merger.

     Escrowed Units.  In connection with the acquisition of William Wilson &
Associates in December 1998, an aggregate of 387,137 partnership units issued by
the Cornerstone Partnership that would have been distributable to the general
partners or managers of the four Wilson-sponsored opportunity funds acquired by
Cornerstone in the transaction were placed in escrow pursuant to an agreement
with certain investors in the opportunity funds. These units would have been
distributed to these general partners and managers under the governing
instruments of the opportunity funds if the units were valued at $17.25 per
unit, the agreed-upon value of the Cornerstone shares and units issued in the
Wilson transaction. The units were placed in escrow pending a determination of
their value during the one-year period beginning on the date the units were
released from the lockup provisions applicable to the units issued in the Wilson
acquisition. Mr. Wilson and other executives of Cornerstone indirectly
beneficially own in the aggregate 319,369 of such escrowed units.

                                       15
<PAGE>   28

FAILURE OF CORNERSTONE STOCKHOLDERS TO APPROVE THE MERGER AGREEMENT MAY REQUIRE,
UNDER LIMITED CIRCUMSTANCES, CORNERSTONE TO PAY TERMINATION FEES AND MAY RESULT
IN A DECREASE IN THE MARKET PRICE OF CORNERSTONE COMMON STOCK

     If the Cornerstone common and preferred stockholders fail to approve the
merger agreement, or if the merger is not completed for any reason, Cornerstone
may be subject to material risks, including the risk that, under limited
circumstances, Cornerstone and Cornerstone Partnership pay up to $100 million in
termination fees and expenses to EOP Partnership, which could lead to a possible
decline in the market price of Cornerstone common stock to the extent current
market prices reflect a market assumption that the merger will be completed. For
a more detailed discussion on termination fees and expenses, see "The Merger
Agreement -- Termination of the Merger Agreement -- Expenses; Termination Fee"
on page 67.

     If, following termination of the merger agreement, the Cornerstone board of
directors determines to seek another merger or business combination, there can
be no assurance that it will be able to find a partner willing to pay an
equivalent or more attractive price than would be provided by Equity Office in
the merger. Under the terms of the merger agreement, before the termination of
the merger agreement, Cornerstone is not permitted to:

     - invite, initiate, solicit or encourage any alternative acquisition
       proposal;

     - except in circumstances expressly permitted under the merger agreement,
       engage in any discussions or negotiations with, or provide any
       confidential or non-public information or data to, any person relating
       to, or that may reasonably be expected to lead to, an alternative
       acquisition proposal;

     - enter into any letter of intent, agreement in principle or agreement
       relating to an alternative acquisition proposal; or

     - publicly propose to agree to do any of the foregoing or otherwise
       facilitate any effort or attempt to make or implement an alternative
       acquisition proposal. See "The Merger Agreement -- No Solicitation by
       Cornerstone" on page 64.

IF THE AGGREGATE FAIR MARKET VALUE OF THE EQUITY OFFICE COMMON SHARES TO BE
ISSUED IN THE MERGER DOES NOT REPRESENT AT LEAST 40% OF THE MERGER CONSIDERATION
AT THE TIME OF THE CLOSING OF THE MERGER, LEGAL OPINIONS REQUIRED TO BE
DELIVERED FOR THE MERGER TO BE COMPLETED COULD NOT BE DELIVERED, IN WHICH CASE
THE MERGER WOULD NOT BE COMPLETED AND THE MARKET PRICE OF CORNERSTONE COMMON
STOCK COULD DECLINE AS A RESULT

     It is a condition to the consummation of the merger that Hogan & Hartson
L.L.P., counsel to Equity Office, and King & Spalding, counsel to Cornerstone,
deliver opinions to Equity Office and Cornerstone, respectively, that the merger
qualifies as a reorganization under section 368(a) of the Internal Revenue Code.
The required tax opinions will not be delivered, and the merger will not be
consummated, unless the aggregate fair market value of the Equity Office common
shares issued in the merger represents at least 40% of the aggregate value of
the total consideration issued by Equity Office in the merger, as of the closing
of the merger and taking into account any cash paid for fractional Equity Office
common shares. For example, if the merger had occurred on             , 2000,
then based on the trading price of Equity Office common shares on that date and
the number of shares of Cornerstone common stock issued and outstanding on that
date, assuming that no holders of Cornerstone Partnership units had converted
their units into Cornerstone common stock immediately before the merger, the
aggregate fair market value of the Equity Office common shares that would have
been issued in the merger would represent approximately      % of the value of
the total consideration paid by Equity Office in the merger. There is no
guarantee, however, that the trading price of the Equity Office common shares
will be high enough at the scheduled closing of the merger to allow counsel to
render the required tax opinions and the merger to be completed.

                                       16
<PAGE>   29

EQUITY OFFICE AND CORNERSTONE MAY NOT BE SUCCESSFULLY INTEGRATED AND INTENDED
BENEFITS OF THE MERGER MAY NOT BE REALIZED, WHICH COULD HAVE A NEGATIVE IMPACT
ON THE MARKET PRICE OF EQUITY OFFICE COMMON SHARES AFTER COMPLETION OF THE
MERGER

     The completion of the merger poses risks for the ongoing operations of
Equity Office, including that:

     - following the merger, Equity Office may not achieve the expected costs
       savings and operating efficiencies;

     - the Cornerstone portfolio may not perform as well as Equity Office
       anticipates;

     - Equity Office may experience difficulties and incur expenses associated
       with the assimilation and retention of Cornerstone non-executive
       employees; and

     - Equity Office may not effectively integrate Cornerstone's operations.

     If Equity Office fails to successfully integrate Cornerstone and/or fails
to realize intended benefits of the merger due to any of the foregoing reasons,
the market price of Equity Office common shares could decline from its market
price at the time of completion of the merger.

                                       17
<PAGE>   30

                   A WARNING ABOUT FORWARD-LOOKING STATEMENTS

     Equity Office and Cornerstone have each made forward-looking statements in
this document, and in documents that are incorporated by reference in this
document, that are subject to risks and uncertainties. Forward-looking
statements include the information concerning possible or assumed future results
of operations of Equity Office and Cornerstone. Also, statements including words
such as "believes," "expects," "anticipates," "intends," "plans," "estimates,"
or similar expressions are forward-looking statements. Many factors, some of
which are discussed elsewhere in this document and in the documents incorporated
by reference in this document, could affect the future financial results of
Equity Office and could cause actual results to differ materially from those
expressed in forward-looking statements contained or incorporated by reference
in this document. Important factors that could cause actual results to differ
materially from the expectations reflected in the forward-looking statements in
this joint proxy statement/prospectus include, among others, the factors
discussed under the caption "Risk Factors" on page 13, changes of a
macroeconomic nature, including changes in the economic conditions affecting
industries in which Equity Office's principal tenants compete, the ability to
timely lease or re-lease square footage at current or anticipated rents, the
ability to achieve economies of scale over time, the demand for tenant services
beyond those traditionally provided by landlords, changes in interest rates,
changes in operating costs, future demand for Equity Office's debt and equity
securities and the ability to complete current and future development projects
on time and on schedule.

     Forward-looking statements are not guarantees of performance. They involve
risks, uncertainties and assumptions. The future results and shareholder values
of Equity Office following completion of the merger may differ materially from
those expressed in these forward-looking statements. Many of the factors that
will determine these results and values are beyond Equity Office's and
Cornerstone's ability to control or predict. For those statements, Equity Office
and Cornerstone claim the protection of the safe harbor for forward-looking
statements contained in the Private Securities Litigation Reform Act of 1995.

                                       18
<PAGE>   31

                       THE EQUITY OFFICE SPECIAL MEETING

DATE, TIME, PLACE AND PURPOSE OF THE EQUITY OFFICE SPECIAL MEETING

     The special meeting of the Equity Office common shareholders is scheduled
to be held on             , 2000, at           a.m., local time, at
               . It may be adjourned or postponed to another date and/or place
for proper purposes. The purpose of the meeting is to consider and vote upon a
proposal to adopt and approve the merger agreement and the merger of Cornerstone
into Equity Office under the merger agreement. The Equity Office common
shareholders also might be asked to vote upon a proposal to adjourn the Equity
Office special meeting for the purpose, among others, of allowing additional
time for the solicitation of additional votes to adopt and approve the merger
agreement. As part of the merger, the Equity Office declaration of trust will be
amended to add a limitation on the ownership by non-U.S. persons of Equity
Office common shares and preferred shares issued in the future.

WHO CAN VOTE

     You are entitled to vote your Equity Office common shares if our records
showed that you held your Equity Office common shares as of the close of
business on             , 2000. At the close of business on that date, a total
of                Equity Office common shares were outstanding and entitled to
vote. Each Equity Office common share has one vote. The enclosed proxy card
shows the number of Equity Office common shares which you are entitled to vote.
Your individual vote is confidential and will not be disclosed to third parties.

VOTING BY PROXY HOLDERS

     If your Equity Office common shares are held by a broker, bank or other
nominee, you will receive instructions from your nominee which you must follow
in order to have your common shares voted. If you hold your Equity Office common
shares in your name as a holder of record, you may instruct the proxy holders
how to vote your Equity Office common shares by using the toll-free telephone
number, the Internet website listed on the proxy card or by signing, dating and
mailing the proxy card in the postage-paid envelope that we have provided to
you. Whichever of these methods you select to transmit your instructions, the
proxy holders will vote your Equity Office common shares in accordance with
those instructions. If you give us a proxy without giving specific voting
instructions, your Equity Office common shares will be voted by the proxy holder
as recommended by our board of trustees.

VOTE BY TELEPHONE

     You can vote your Equity Office common shares by telephone by dialing the
toll-free telephone number, at no cost to you, printed on your proxy card.
Telephone voting is available 24 hours a day until 3:00 p.m. Eastern Time, on
            ,             , 2000. Easy-to-follow voice prompts allow you to vote
your Equity Office common shares and confirm that your instructions have been
properly recorded. Our telephone voting procedures are designed to authenticate
shareholders by using individual control numbers. If you vote by telephone, you
do not need to return your proxy card. If you are located outside the U.S. and
Canada, you should use the collect calling option printed on your proxy card.

VOTE BY INTERNET

     You can also choose to vote via the Internet. The website for Internet
voting is printed on your proxy card. Internet voting is available 24 hours a
day until 3:00 p.m., Eastern Time on             ,             , 2000. As with
telephone voting, you will be given the opportunity to confirm that your
instructions have been properly recorded. If you vote via the Internet, you do
not need to return your proxy card.

                                       19
<PAGE>   32

VOTE BY MAIL

     If you choose to vote by mail, simply mark your proxy card, sign and date
it, and return it to EquiServe in the postage-paid envelope provided.

REQUIRED VOTE

     Adoption and approval of the merger agreement and the merger require the
affirmative vote of the holders of at least a majority of the outstanding Equity
Office common shares entitled to vote at the Equity Office special meeting. The
vote of the holders of Equity Office's preferred shares is not required for
adoption and approval of the merger agreement and the merger.

VOTING ON OTHER MATTERS

     We are not now aware of any other matters to be presented at the special
meeting except for those described in this joint proxy statement/prospectus. If
any other matters not described in this joint proxy statement/prospectus are
properly presented at the meeting, the proxy holders will use their own judgment
to determine how to vote your Equity Office common shares. If the meeting is
adjourned or postponed, your Equity Office common shares may be voted by the
proxy holders on the new meeting date as well, unless you have revoked your
proxy instructions before such date.

HOW YOU MAY REVOKE YOUR PROXY INSTRUCTIONS

     To revoke your proxy instructions, you must (a) so advise our secretary,
Stanley M. Stevens, c/o Equity Office Properties Trust, Two North Riverside
Plaza, Suite 2100, Chicago, IL 60606 in writing received before your Equity
Office common shares have been voted by the proxy holder at the meeting, (b)
deliver to our secretary before the date of the meeting later proxy
instructions, or (c) attend the meeting and vote your Equity Office common
shares in person.

HOW VOTES ARE COUNTED

     The special meeting will be held if a majority of the outstanding Equity
Office common shares entitled to vote is represented at the meeting. If you have
returned valid proxy instructions or attend the meeting in person, your Equity
Office common shares will be counted for the purpose of determining whether
there is a quorum, even if you wish to abstain from voting on some or all
matters introduced at the meeting. If you hold your Equity Office common shares
through a broker, bank or other nominee, the nominee may only vote the Equity
Office common shares which it holds for you in accordance with your
instructions. If it has not received your instructions by the 10th day prior to
the meeting, the nominee may not vote on the merger agreement and the merger,
which would result in a "broker non-vote" on the merger agreement and the
merger. Broker non-votes count for quorum purposes, but abstentions and broker
non-votes will have the same effect as a negative vote on matters presented at
the special meeting.

COST OF THIS PROXY SOLICITATION

     Equity Office will pay the cost of its proxy solicitation. In addition to
soliciting proxies by mail, we expect that certain of our employees will solicit
Equity Office common shareholders personally and by telephone. None of these
employees will receive any additional or special compensation for doing this. We
will, upon request, reimburse brokers, banks and other nominees for their
reasonable expenses in sending proxy material to their principals and obtaining
their proxies.

ATTENDING THE EQUITY OFFICE SPECIAL MEETING

     If you are a holder of record of Equity Office common shares and you plan
to attend the Equity Office special meeting, please indicate this when you vote.
If you are a beneficial owner of Equity Office common shares held by a bank or
broker, you will need proof of ownership to be admitted to the meeting. A recent
brokerage statement or letter from a bank or broker are examples of proof of
ownership. If you

                                       20
<PAGE>   33

want to vote in person your Equity Office common shares held in street name, you
will have to get a proxy in your name from the registered holder.

LIST OF EQUITY OFFICE COMMON SHAREHOLDERS

     A list of Equity Office common shareholders entitled to vote at the Equity
Office special meeting will be available at the Equity Office special meeting
and for ten days before the meeting between the hours of 8:45 a.m. and 4:30
p.m., Central Time, at our corporate offices located at Two North Riverside
Plaza, Suite 2100, Chicago, Illinois 60606, by contacting Stanley M. Stevens,
the secretary of Equity Office.

                                       21
<PAGE>   34

                        THE CORNERSTONE SPECIAL MEETING

DATE, TIME, PLACE AND PURPOSE OF THE CORNERSTONE SPECIAL MEETING

     The special meeting of the Cornerstone stockholders is scheduled to be held
on             , 2000, at      a.m., local time, at           . It may be
adjourned or postponed to another date and/or place for proper purposes. The
purpose of the meeting is to consider and vote upon a proposal to approve the
merger agreement. The Cornerstone stockholders also might be asked to vote upon
a proposal to adjourn the Cornerstone special meeting for the purpose, among
others, of allowing additional time for the solicitation of additional votes to
approve the merger agreement.

WHO CAN VOTE

     You are entitled to vote your shares of Cornerstone common stock and
Cornerstone 7% cumulative convertible preferred stock if our records showed that
you held your shares as of the close of business on             , 2000. At the
close of business on that date, a total of                shares of Cornerstone
common stock and 3,030,303 shares of Cornerstone 7% cumulative convertible
preferred stock were outstanding and entitled to vote, respectively. Each share
of Cornerstone common and preferred stock has one vote. The Cornerstone
preferred stock will vote separately as a class on the merger agreement. The
enclosed proxy card shows the number of shares of Cornerstone common stock or
Cornerstone preferred stock which you are entitled to vote. Your individual vote
is confidential and will not be disclosed to third parties.

VOTING BY PROXY HOLDERS

     If your shares of Cornerstone stock are held by a broker, bank or other
nominee, you will receive instructions from your nominee which you must follow
in order to have your common shares voted. If you hold your shares of
Cornerstone stock in your name as a holder of record, you may instruct the proxy
holders how to vote your shares of Cornerstone stock by signing, dating and
mailing the proxy card in the postage-paid envelope that we have provided to
you. The proxy holders will vote your shares of Cornerstone common stock in
accordance with those instructions. If you give us a proxy without giving
specific voting instructions, your shares of Cornerstone common stock will be
voted by the proxy holder as recommended by our board of directors.

VOTING AND ELECTION PROCEDURES

     Proxy cards.  To vote, simply mark your proxy card, sign and date it, and
return it to                in the postage-paid envelope provided. FOR HOLDERS
OF SHARES OF CORNERSTONE COMMON STOCK IN GERMANY: IN THE EVENT THE BANK HOLDING
YOUR CORNERSTONE COMMON STOCK HAS NOT PROVIDED YOU WITH A RETURN ENVELOPE,
PLEASE RETURN YOUR PROXY DIRECTLY TO YOUR DEPOSITORY BANK.

     Forms of Election for Holders of Cornerstone Common Stock.  In addition to
a proxy card, a form of election will be mailed to each holder of Cornerstone
common stock. In order to be effective, a form of election must be properly
completed, signed and submitted to the exchange agent listed in the instructions
to the form of election by the close of business on             , 2000. The form
of election must be accompanied by the certificates representing the shares of
Cornerstone common stock as to which the election is made, or by an appropriate
guarantee of delivery of such certificates. If a Cornerstone stockholder does
not submit a properly completed form of election which is received by the
exchange agent before             , 2000 at the address provided in the
instructions to the form of election, or if such Cornerstone stockholder does
not include the certificates representing the shares of Cornerstone common stock
as to which the election is made or an appropriate guarantee of delivery of such
certificates, then such Cornerstone stockholder will be deemed not to have made
an election.

     If a Cornerstone stockholder properly completes, signs and submits a form
of election to the exchange agent with such stockholder's certificates
representing the shares of Cornerstone common stock as to which such election is
made, or with an appropriate guarantee of delivery of such certificates, and the
merger is
                                       22
<PAGE>   35

completed, each share of Cornerstone common stock covered by such form of
election will be exchanged for the merger consideration as indicated on the form
of election and in accordance with the merger agreement.

     EQUITY OFFICE AND CORNERSTONE INTEND TO COMPLETE THE MERGER IMMEDIATELY
AFTER THE SPECIAL MEETINGS. SUCH COMPLETION, HOWEVER, COULD BE DELAYED BY EQUITY
OFFICE AND CORNERSTONE IN ORDER THAT OTHER CLOSING CONDITIONS CAN BE SATISFIED.
ACCORDINGLY, ONCE A FORM OF ELECTION HAS BEEN DELIVERED TOGETHER WITH
CERTIFICATES OR APPROPRIATE GUARANTEES OF DELIVERY THEREFOR, A CORNERSTONE
STOCKHOLDER WILL BE UNABLE TO SELL, OR OTHERWISE TRANSFER, SUCH STOCKHOLDER'S
SHARES FOR AN INDEFINITE PERIOD OF TIME.

REQUIRED VOTE

     Approval of the merger agreement requires the affirmative vote of (a) the
holders of at least a majority of the outstanding shares of Cornerstone Common
Stock and (b) the holders of at least two-thirds of the outstanding shares of
Cornerstone 7% cumulative convertible preferred stock.

VOTING ON OTHER MATTERS

     We are not now aware of any other matters to be presented at the special
meeting except for those described in this joint proxy statement/prospectus. If
any other matters not described in this joint proxy statement/prospectus are
properly presented at the meeting, the proxy holders will use their own judgment
to determine how to vote your shares of Cornerstone stock. If the meeting is
adjourned or postponed, your shares of Cornerstone stock may be voted by the
proxy holders on the new meeting date as well, unless you have revoked your
proxy instructions before such date.

HOW YOU MAY REVOKE YOUR PROXY INSTRUCTIONS OR FORM OF ELECTION

     Revocation of a Proxy.  To revoke your proxy instructions, you must (a) so
advise our secretary, Thomas P. Loftus, c/o Cornerstone Properties Inc., 126
East 56th Street, New York, New York 10022 in writing received before your
shares of Cornerstone stock have been voted by the proxy holders at the meeting,
(b) deliver to Mr. Loftus before the date of the meeting proxy instructions
dated after your initial proxy instructions, or (c) attend the meeting and vote
your shares of Cornerstone stock in person.

     Revocation of a Form of Election.  To revoke a form of election previously
delivered to the exchange agent, you must provide written notice to the exchange
agent, or withdraw your certificates that have been previously deposited with
the exchange agent, at or before 5:00 p.m., Eastern Time, on             , 2000.
In order to withdraw a revocation, you must deliver written notice of such
withdrawal to the exchange agent before 5:00 p.m., Eastern Time, on
            , 2000. No revocation or withdrawal will be effective after the
times described above.

HOW VOTES ARE COUNTED

     The special meeting will be held if 20% of the outstanding shares of
Cornerstone stock entitled to vote are represented at the meeting. If you have
returned valid proxy instructions or attend the meeting in person, your shares
of Cornerstone stock will be counted for the purpose of determining whether
there is a quorum, even if you wish to abstain from voting on some or all
matters introduced at the meeting. If you hold your shares of Cornerstone stock
through a broker, bank or other nominee, the nominee may only vote the shares of
Cornerstone stock which it holds for you in accordance with your instructions.
If it has not received your instructions by the 10th day prior to the meeting,
the nominee may not vote on the merger, which would result in a "broker
non-vote" on the merger. Broker non-votes count for quorum purposes, but
abstentions and broker non-votes will have the same effect as a negative vote on
matters presented at the special meeting.

                                       23
<PAGE>   36

COST OF THIS PROXY SOLICITATION

     Cornerstone will pay the cost of its proxy solicitation. In addition to
soliciting proxies by mail, we expect that certain of our officers or other
representatives of Cornerstone or The Bank of New York, Cornerstone's transfer
agent, will solicit Cornerstone stockholders personally and by telephone. None
of these employees or representatives will receive any additional or special
compensation for doing this. We will, upon request, reimburse brokers, banks and
other nominees for their reasonable expenses in sending proxy material to their
principals and obtaining their proxies.

ATTENDING THE CORNERSTONE SPECIAL MEETING

     If you are a holder of record of Cornerstone stock and you plan to attend
the Cornerstone special meeting, please indicate this when you vote. If you are
a beneficial owner of Cornerstone stock held by a bank or broker, you will need
proof of ownership to be admitted to the meeting. A recent brokerage statement
or letter from a bank or broker are examples of proof of ownership. If you want
to vote in person your Cornerstone stock held in street name, you will have to
get a proxy in your name from the registered holder.

LIST OF CORNERSTONE STOCKHOLDERS

     A list of Cornerstone stockholders entitled to vote at the Cornerstone
special meeting will be available at the Cornerstone special meeting and for ten
days before the meeting between the hours of      a.m. and      p.m., Eastern
Time, at our corporate offices located at 126 East 56th Street, New York, New
York 10022, by contacting Thomas P. Loftus, the secretary of Cornerstone.

                                       24
<PAGE>   37

                                   THE MERGER

STRUCTURE OF THE MERGERS

     General.  The merger agreement contemplates the following two-step
transaction:

     - the partnership merger, whereby Cornerstone Partnership will merge with
       and into EOP Partnership and Cornerstone Partnership will cease to exist;
       followed by

     - the merger, whereby Cornerstone will merge with and into Equity Office
       and Cornerstone will cease to exist.

     Merger Consideration.  In the partnership merger, holders of class A units
of limited partnership interest in Cornerstone Partnership who have not elected
to redeem their units for shares of Cornerstone common stock will receive, for
each unit issued and outstanding immediately before the partnership merger,
0.7009 of a class A unit of limited partnership interest in EOP Partnership, as
described under "-- The Partnership Merger" on page 44. No cash will be paid to
holders of Cornerstone Partnership units in connection with the partnership
merger.

     In the merger:

     - holders of Cornerstone common stock may elect to receive either $18.00
       per share in cash, without interest, or 0.7009 of an Equity Office common
       share, subject to proration if the cash election is oversubscribed or
       undersubscribed; and

     - holders of Cornerstone 7% cumulative convertible preferred stock will
       receive $18.00 per share in cash, without interest, together with accrued
       and unpaid dividends through the date of the closing of the merger.

     Under the merger agreement, a total of 58,551,525 shares of Cornerstone
common stock will be converted in the merger into the right to receive $18.00
per share in cash, without interest, and the number of shares of Cornerstone
common stock issued and outstanding immediately before the merger in excess of
58,551,525 shares will be converted into the right to receive Equity Office
common shares based on the 0.7009 exchange ratio.

     Because the aggregate cash portion of the merger consideration to the
Cornerstone common stockholders is fixed at approximately $1.054 billion, or
$18.00 times 58,551,525 shares, and the number of Equity Office common shares
issuable in the merger is fixed at 0.7009 times the number of shares of
Cornerstone common stock issued and outstanding immediately before the closing
of the merger in excess of 58,551,525 shares, the amount of cash and the number
of Equity Office common shares that each Cornerstone stockholder will receive in
the merger may be subject to proration based on the elections made by the
Cornerstone common stockholders.

     Cash will be paid instead of fractional Equity Office common shares. This
cash will be in addition to the approximately $1.054 billion of cash required to
be paid to Cornerstone common stockholders as part of the merger consideration.
The merger will be consummated immediately after the consummation of the
partnership merger.

     Holders of class A units of limited partnership interest in Cornerstone
Partnership will have the right to have all or any portion of their Cornerstone
Partnership units redeemed by Cornerstone for shares of Cornerstone common stock
immediately before the partnership merger. Such holders would then participate
in the merger as holders of Cornerstone common stock. If the former holders of
Cornerstone Partnership units elect to receive cash in the merger, the cash
available to existing Cornerstone common stockholders in the merger will be
reduced. See "-- The Partnership Merger."

BACKGROUND OF THE MERGER

     In pursuing their strategies for enhancing shareholder value, each of
Equity Office and Cornerstone regularly consider opportunities for acquisitions,
joint ventures and other significant transactions.
                                       25
<PAGE>   38

     Throughout the second and third quarters of 1999, senior management of
Cornerstone met on several occasions to determine whether or not Cornerstone
should undertake a comprehensive review of the strategic alternatives available
to the company. These discussions were included as part of Cornerstone's regular
strategic planning sessions. These discussions involved a review of the general
status of the industry, the industry's financial performance and trends in the
capital markets, as well as Cornerstone's financial performance and outlook, the
integration of the William Wilson & Associates merger completed in December
1998, Cornerstone's development pipeline and its asset disposition program. In
addition to an evaluation of Cornerstone's current strategy, this discussion
included an assessment of the potential for a strategic combination with another
office REIT.

     At the regularly scheduled Cornerstone board of directors meeting held on
September 28, 1999, John S. Moody, the president and chief executive officer of
Cornerstone, reviewed with the board the current strategic direction of the
company. In his review, Mr. Moody noted the challenges that faced Cornerstone as
a publicly-traded REIT in the current market environment, particularly in
expanding its development capabilities outside of Northern California,
identifying significant acquisition candidates and accessing public equity
markets that were not receptive to REITs raising capital because of the relative
underperformance of REIT stocks in 1998 and 1999. At the September 28, 1999
Cornerstone board meeting, Lazard Freres & Co. LLC presented a financial
analysis of various strategic alternatives, including a more detailed analysis
of a hypothetical business combination of a specific potential merger partner
that had been previously identified to Lazard by Cornerstone's management team
as a logical candidate for a business combination. The board decided only to
explore the possibility of a merger transaction with this particular party. The
Cornerstone board authorized the engagement of Lazard to act as financial
advisor to Cornerstone and authorized management and Lazard to contact such
potential strategic partner to determine its interest in a possible business
combination.

     In October and November of 1999, representatives of Cornerstone and Lazard
met on several occasions with representatives of this potential merger partner
and provided financial and operating data regarding Cornerstone to assist the
other party in assessing a possible merger.

     In early December 1999, Mr. Moody telephoned Timothy H. Callahan, the
president and chief executive officer of Equity Office, to congratulate Mr.
Callahan on Equity Office's completion of the Lend Lease transaction. During the
conversation, Messrs. Callahan and Moody discussed the general state of the real
estate industry and public market valuations of real estate investment trusts.
Mr. Moody and Mr. Callahan also agreed during the call that public market
valuations of real estate investment trusts at the time made business
combinations very difficult. No specific acquisition proposal was discussed
between Messrs. Moody and Callahan during such conversation. This conversation
was one of several similar conversations Messrs. Moody and Callahan had over the
last several years about the real estate industry in general and the prospects
for consolidation among office real estate investment trusts in particular.
Other executives of Equity Office and Cornerstone also speak with each other
from time to time, as well as with executives of other real estate companies,
regarding the industry in general.

     On December 9, 1999, the Cornerstone board met to consider a preliminary
proposal from the party identified at the September 28, 1999 board meeting.
Lazard made a presentation regarding the financial aspects of the proposal. The
Cornerstone board discussed the proposal, asked questions of Lazard and
management and concluded that the proposal did not provide adequate value to the
stockholders of Cornerstone. Based on the advice of management, the board also
concluded that further discussions were not likely to result in a more
attractive proposal. The Cornerstone board also discussed the possibility of
initiating discussions with Equity Office, but determined that public market
valuations of real estate investment trusts at the time made pursuing a business
combination very difficult. Accordingly, the board terminated the engagement of
Lazard and directed management to cease any further discussions of a potential
business combination with the potential strategic partner and not to initiate
discussions with any other party, including Equity Office.

     During early January, Mr. Moody and Mr. Callahan again exchanged telephone
calls during which they agreed that the public markets had improved
significantly for real estate investment trusts over the

                                       26
<PAGE>   39

past month. Mr. Callahan further expressed an interest in meeting with Mr. Moody
regarding a possible business combination between Equity Office and Cornerstone.
Mr. Moody suggested to Mr. Callahan that he should come to New York and meet
with him and with William Wilson III, Cornerstone's chairman of the board. On
January 10, 2000, Messrs. Callahan, Wilson and Moody met in New York and
discussed the benefits of a possible acquisition of Cornerstone by Equity
Office. No negotiations took place during this meeting. However, both sides
agreed to pursue additional discussions. On January 13, 2000, Equity Office and
Cornerstone executed a mutual confidentiality agreement and agreed to exchange
non-public information. On January 18, 2000, Mr. Callahan contacted J.P. Morgan,
Equity Office's financial advisor, and requested that J.P. Morgan commence its
research and analysis of Cornerstone.

     On January 19, 2000, Messrs. Wilson, Moody and Callahan and Samuel Zell,
Equity Office's chairman of the board, met in New York. At this meeting, Messrs.
Zell and Callahan expressed Equity Office's interest in acquiring Cornerstone in
a cash and stock merger, with Cornerstone common shares valued at $17.50 per
share, consisting of $6.56 in cash and $10.94 in Equity Office common shares,
subject to completion of due diligence. No negotiations took place during this
meeting, and Mr. Moody indicated that he would have to ask the Cornerstone board
for guidance as to whether to engage in further discussions.

     On January 20, 2000, the Cornerstone board met at a regularly scheduled
meeting. Mr. Moody reported to the Cornerstone board Equity Office's offer to
acquire Cornerstone. At this meeting, the Cornerstone board did not authorize
management to enter into formal discussions with Equity Office, but decided to
engage Lazard to assist in an evaluation of the merits of a possible acquisition
of Cornerstone by Equity Office. Over the next ten days, representatives of
Lazard met with representatives of Equity Office and J. P. Morgan to exchange
financial and operating data relating to Cornerstone and Equity Office and to
receive more information about the terms and structure of a possible
transaction.

     On January 31, 2000, the Cornerstone board met again to consider the Equity
Office proposal. Lazard reviewed the terms of the proposed transaction and
provided a financial analysis of the transaction and other strategic
alternatives. At the meeting, the Cornerstone board discussed: (a) the financial
terms of the proposed transaction with Equity Office; (b) the potential benefits
of such a business combination; (c) the advantages and disadvantages of the
transaction compared with Cornerstone's other strategic alternatives; (d) the
potential conditions to the completion of the transaction, including approvals
of Cornerstone stockholders and limited partners of Cornerstone Partnership; and
(e) the tax consequences of the proposed transaction. At the conclusion of the
meeting, the Cornerstone board authorized management and Lazard to commence
formal negotiations with Equity Office with a principal objective of achieving a
higher price.

     At the regularly scheduled Equity Office board of trustees meeting held on
February 1, 2000, Mr. Callahan updated the board on the discussions with Messrs.
Wilson and Moody and Lazard regarding a possible acquisition of Cornerstone by
Equity Office. At this meeting, various members of the board expressed their
support for such a transaction if it could be accomplished along the lines
described at the meeting.

     During the week of January 31 through February 5, senior management of
Equity Office and Cornerstone, together with their respective financial
advisors, engaged in negotiations regarding the financial terms of the proposed
transaction. On February 5, Lazard asked Equity Office to submit its best and
final proposal for a merger transaction no later than the following morning. On
February 5, Equity Office submitted a revised merger proposal, which valued
Cornerstone common stock at $18.00 per share, consisting of $7.06 in cash and
$10.94 in Equity Office common shares, with the exchange ratio to be fixed based
upon the 10-day average closing share prices of Cornerstone and Equity Office
ending at the close of trading on Friday, February 4.

     The Cornerstone board met by telephone on the morning of Sunday, February 6
to consider the revised Equity Office proposal. Prior to that meeting, senior
management of Cornerstone and representatives of Lazard discussed by telephone
the terms of the proposal with representatives of PGGM, Cornerstone's largest
stockholder. After considering the proposal, PGGM indicated to management of
                                       27
<PAGE>   40

Cornerstone and Lazard that it was prepared to approve the revised proposal,
subject to satisfactory negotiation of the definitive agreements. During the
Cornerstone board meeting, Lazard reviewed the negotiations of the past week and
reviewed the financial terms of the revised proposal. At the conclusion of the
meeting, the Cornerstone board authorized management to negotiate a definitive
merger agreement and related documentation with Equity Office.

     From February 7, 2000 through February 10, 2000, management of Equity
Office and Cornerstone, and their respective legal and financial advisors, met
to negotiate the terms of the merger agreement, the mergers and the ancillary
documents, including the cash/share election mechanics, a stock option agreement
with the holders of the outstanding shares of Cornerstone preferred stock (see
"The Merger Agreement -- Preferred Stock Option Agreement" on page 73) and the
voting agreements. Each of the parties also completed its due diligence review
of the other party.

     On the evening of February 10, 2000, the Equity Office board held a special
meeting at which members of management and representatives of J.P. Morgan and
Equity Office's outside counsel, Hogan & Hartson L.L.P., were present in person
or by conference telephone call. The special meeting was held in order for the
Equity Office board to consider and formally act upon the proposed acquisition
of Cornerstone. At this meeting, Equity Office's senior management reviewed with
the board financial and business terms of the proposed transaction, including
the need for Equity Office to finance the cash portion of the proposed merger
consideration, and the results of Equity Office's due diligence review. J.P.
Morgan then presented its financial analysis of the proposed merger, and
delivered to the Equity Office board its oral opinion that the merger
consideration to be paid by Equity Office in the transaction was fair, from a
financial point of view, to Equity Office. Equity Office's legal counsel then
made a presentation to the Equity Office board in which it explained the
material terms of the proposed merger agreement and related agreements,
including closing conditions, termination rights and provisions regarding
break-up fees and termination expenses, and briefed the board on open items.

     Following these presentations, the Equity Office trustees asked numerous
questions of management and its legal counsel and financial advisors and
discussed at length the issues raised by the presentations. At the conclusion of
these discussions, the Equity Office board approved the merger and the merger
agreement on substantially the terms discussed at the meeting, and authorized
management to complete negotiations of, and execute, the merger agreement,
subject to final approval of the transaction terms by the executive committee of
the board and receipt of J.P. Morgan's written fairness opinion.

     On the evening of February 10, 2000, the Cornerstone board met to review
and discuss the terms of the proposed merger agreement and related agreements.
Representatives of Lazard and Cornerstone's outside counsel, King & Spalding,
and senior management reviewed with the Cornerstone board the results of the
company's financial, business and legal due diligence examinations and the terms
of the merger agreement, the voting agreements, the stock option agreement and
the severance and non-competition arrangements with management of Cornerstone.
Lazard presented a financial analysis of the merger and indicated that, upon
completion of definitive documentation relating to the merger, it would deliver
its opinion that the consideration to be received in the merger was fair, from a
financial point of view, to the holders of Cornerstone common stock. Counsel for
Cornerstone made a presentation to the Cornerstone board in which it explained
the material terms of the proposed merger agreement and related agreements,
including closing conditions, termination rights and provisions regarding
break-up fees and termination expenses, and highlighted the few issues that were
as yet unresolved in the merger agreement. The Cornerstone board directed
management to continue negotiations through the evening on the definitive merger
agreement.

     Throughout the night of February 10, 2000, representatives of Equity Office
and Cornerstone and their respective legal counsel negotiated the remaining
unresolved terms of the merger agreement.

     On the morning of February 11, 2000, members of the executive committee of
the Equity Office board of trustees approved the final terms of the merger
agreement and the ancillary agreements and J.P. Morgan delivered its written
fairness opinion.

                                       28
<PAGE>   41

     On the morning of February 11, 2000, the Cornerstone board reconvened to
review and discuss the terms of the definitive merger agreement and related
agreements. Cornerstone's counsel reviewed the resolution of the issues that had
been unresolved as of the previous evening's meeting, and reviewed again the
terms of the definitive merger agreement, the voting agreements, the option
agreement with Deutsche Bank and the severance arrangements with management.
After discussion of the foregoing, the Cornerstone board determined that the
merger was fair to and in the best interests of Cornerstone and the holders of
shares of Cornerstone stock, that the merger agreement and the transactions
contemplated thereby were advisable and that the board should recommend that the
stockholders of Cornerstone approve the merger agreement, and approved and
adopted the merger agreement, the mergers, the voting agreements, the option
agreement and related matters.

     Immediately after the Cornerstone board meeting on the morning of February
11, the parties executed the merger agreement, the option agreement, their
respective voting agreements and issued a joint press release announcing the
transaction.

EQUITY OFFICE'S REASONS FOR THE MERGER; RECOMMENDATION OF THE EQUITY OFFICE
BOARD

     The Equity Office board of trustees has approved the merger and the merger
agreement. The Equity Office board believes that the terms of the merger, the
merger agreement and the other transactions contemplated thereby are advisable
and in the best interests of Equity Office and its shareholders. Accordingly,
the Equity Office board recommends that Equity Office common shareholders
approve the merger and the merger agreement.

     In its deliberations with respect to the merger and the merger agreement,
the Equity Office board consulted with Equity Office's management and the
financial and legal advisors to Equity Office. The factors considered by the
Equity Office board include those described below. While all of these factors
were considered by the Equity Office board, the board did not make
determinations with respect to each factor. Rather, the board made its judgment
with respect to the merger and the merger agreement based on the total mix of
information available to it, and the judgments of individual trustees may have
been influenced to a greater or lesser degree by their individual views with
respect to different factors.

Positive Factors Considered by the Equity Office Board

     In making its determination with respect to the merger and the merger
agreement, the Equity Office board considered the following material positive
factors:

     - Cornerstone's class A office portfolio will enhance Equity Office's
       existing presence in such top U.S. markets as Boston, San Francisco,
       Washington, D.C., Seattle and Atlanta, all of which are continuing to
       experience significant job growth and many of which have significant
       impediments to new office supply;

     - The Equity Office board believes that the merger presents Equity Office
       with the opportunity to acquire a significant portfolio of well-located,
       high-quality office properties. Cornerstone currently owns or has
       interests in approximately 82 class A office properties, which comprise
       nearly 18 million rentable square feet;

     - The merger, by increasing the number of office properties operated by
       Equity Office by approximately 24% based on square footage, will solidify
       Equity Office's position as the largest publicly-traded office REIT in
       the United States. On a combined basis, Equity Office will have an equity
       market capitalization of approximately $8.9 billion, assuming a share
       price of $25.68125, which equaled Equity Office's average closing share
       price for the ten business days from January 24, 2000 through February 4,
       2000, and a total market capitalization of approximately $18.7 billion;

     - The Equity Office board believes that the merger should generate
       efficiencies through anticipated cost savings and reductions in expenses;

                                       29
<PAGE>   42

     - The Equity Office board believes that the merger will allow Equity Office
       to better meet the needs of its tenants by offering a fuller range of
       increased space options;

     - The Equity Office board believes that Equity Office should be able to
       manage Cornerstone's office properties efficiently following the merger
       because of the significant geographic overlap of Equity Office's and
       Cornerstone's office portfolios; approximately two-thirds of
       Cornerstone's assets are in Equity Office's top eight core markets;

     - Cornerstone will provide Equity Office with an existing development
       pipeline in the San Francisco Bay area, which has significant supply
       contraints;

     - The amount of cash and the exchange ratio are fixed and not subject to
       adjustment; and

     - The opinion, analyses and presentations of J.P. Morgan described under
       "Opinion of Equity Office's Financial Advisor" below, including the
       opinion of J.P. Morgan to the effect that, as of the date of such
       opinion, and based upon and subject to the matters stated therein, the
       consideration to be paid by Equity Office in the merger is fair, from a
       financial point of view, to Equity Office.

Negative Factors Considered by the Equity Office Board

     The Equity Office board of trustees also considered the following
potentially negative factors in its deliberations concerning the merger and the
merger agreement:

     - The need for Equity Office to obtain financing for the cash portion of
       the merger consideration and the resulting increase in the level of
       borrowings of Equity Office and exposure to interest rate risk;

     - The risk that the anticipated benefits of the merger to Equity Office and
       its shareholders may not be realized as a result of possible changes in
       the real estate market in general, the inability to achieve the
       anticipated cost savings and reduction in expenses and other potential
       difficulties in integrating the two companies and their respective
       operations;

     - The significant cost involved in connection with consummating the merger
       and the substantial management time and effort required to effect the
       merger and integrate the businesses of Equity Office and Cornerstone; and

     - The risk that the merger might not be completed based upon the failure to
       satisfy covenants or closing conditions.

     In view of the wide variety of factors considered by the Equity Office
board, the Equity Office board did not quantify or otherwise attempt to assign
relative weights to the specific factors that it considered in making its
determination. In the view of the Equity Office board, however, the potentially
negative factors considered by it were not sufficient, either individually or
collectively, to outweigh the positive factors considered in its deliberations
relating to the merger and the merger agreement.

     The Equity Office board of trustees has determined that the merger is in
the best interests of Equity Office and its shareholders, has approved the
merger agreement and the merger and recommends that Equity Office shareholders
vote to adopt and approve the merger agreement and the merger at the special
meeting.

OPINION OF EQUITY OFFICE'S FINANCIAL ADVISOR

     At the meeting of the board of trustees of Equity Office on February 10,
2000, J.P. Morgan rendered its oral opinion to the board of trustees of Equity
Office that, as of such date, the consideration to be paid by Equity Office in
the proposed merger of Cornerstone into Equity Office was fair from a financial
point of view to Equity Office. J.P. Morgan has confirmed its February 10, 2000
oral opinion by delivering its written opinion to the board of trustees of
Equity Office, dated February 11, 2000, that, as of such date,

                                       30
<PAGE>   43

the consideration to be paid by Equity Office in the proposed merger was fair
from a financial point of view to Equity Office. J.P. Morgan has not been
requested to, and will not, unless requested by Equity Office, update its
opinion before the closing of the merger. No limitations were imposed by Equity
Office's board of trustees upon J.P. Morgan with respect to the investigations
made or procedures followed by it in rendering its opinion.

     THE FULL TEXT OF THE WRITTEN OPINION OF J.P. MORGAN DATED FEBRUARY 11,
2000, WHICH SETS FORTH THE ASSUMPTIONS MADE, MATTERS CONSIDERED AND LIMITS ON
THE REVIEW UNDERTAKEN, IS ATTACHED AS ANNEX B TO THIS JOINT PROXY
STATEMENT/PROSPECTUS AND IS INCORPORATED HEREIN BY REFERENCE. EQUITY OFFICE
COMMON SHAREHOLDERS ARE URGED TO READ THE OPINION IN ITS ENTIRETY. J.P. MORGAN'S
WRITTEN OPINION, ADDRESSED TO THE BOARD OF TRUSTEES OF EQUITY OFFICE, IS
DIRECTED ONLY TO THE CONSIDERATION TO BE PAID IN CONNECTION WITH THE MERGER AND
DOES NOT CONSTITUTE A RECOMMENDATION TO ANY SHAREHOLDER OF EQUITY OFFICE AS TO
HOW SUCH SHAREHOLDER SHOULD VOTE AT THE EQUITY OFFICE SPECIAL MEETING. THE
SUMMARY OF THE OPINION OF J.P. MORGAN SET FORTH IN THIS JOINT PROXY
STATEMENT/PROSPECTUS IS QUALIFIED IN ITS ENTIRETY BY REFERENCE TO THE FULL TEXT
OF SUCH OPINION.

     In arriving at its opinion, J.P. Morgan, among other things:

     - reviewed the merger agreement;

     - reviewed the audited financial statements for the twelve months ended
       December 31, 1998 and the unaudited financial statements for the nine
       months ended September 30, 1999 of Equity Office and Cornerstone;

     - reviewed current and historical market prices of Equity Office common
       shares and Cornerstone common stock;

     - reviewed publicly available information concerning the business of
       Cornerstone and of other companies engaged in businesses comparable to
       those of Cornerstone, and the reported market prices for other companies'
       securities deemed comparable;

     - reviewed publicly available terms of transactions involving companies
       comparable to Cornerstone and the consideration received for such
       companies;

     - reviewed the terms of other business combinations deemed relevant by J.P.
       Morgan;

     - reviewed internal financial analyses and estimates of budgeted 2000
       through 2004 funds from operations and net operating income prepared by
       Equity Office and Cornerstone and their respective management teams;

     - reviewed agreements with respect to outstanding indebtedness or
       obligations of Equity Office and Cornerstone;

     - held discussions with members of the management of Equity Office and
       Cornerstone with respect to various aspects of the merger, and the past
       and current business operations of Equity Office and Cornerstone, the
       financial condition and future prospects and operations of Equity Office
       and Cornerstone and the effects of the merger on the financial condition
       and future prospects of Equity Office and Cornerstone, and other matters
       believed necessary or appropriate to J.P. Morgan's inquiry; and

     - reviewed such other financial studies and analyses and considered such
       other information as J.P. Morgan deemed appropriate for the purposes of
       its opinion.

     J.P. Morgan relied upon and assumed, without independent verification, the
accuracy and completeness of all information that was publicly available or that
was furnished to it by Equity Office and Cornerstone or otherwise reviewed by
J.P. Morgan, and J.P. Morgan has not assumed any responsibility or liability
therefor. J.P. Morgan has not conducted any valuation or appraisal of any assets
or liabilities, nor have any valuations or appraisals been provided to J.P.
Morgan. In relying on financial analyses and forecasts provided to it, J.P.
Morgan has assumed that they have been reasonably prepared based on

                                       31
<PAGE>   44

assumptions reflecting the best currently available estimates and judgments by
management as to the expected future results of operations and financial
condition of Equity Office and Cornerstone to which such analyses or forecasts
relate. J.P. Morgan has also assumed that the merger will have the tax
consequences described in discussions with representatives of Equity Office.

     No representation or warranty was made by any party with respect to the
internal financial analysis and estimates referred to above. Financial
projections are subject to contingencies beyond management's control and
realization of the projections depends on numerous factors, including among
other things, the cost of integrating the companies, the completion of pending
developments, the actual cost in relation to such projects and decisions by
management to modify business plans to address changing needs and a changing
operating environment. All material events and circumstances cannot be predicted
and unanticipated events and circumstances are likely to occur. Accordingly,
there may be differences between the projected results of operations and the
actual results of operations of the respective companies, and such differences
could be material. In the event that the financial projections prove to be
materially different, the conclusions reached in the opinion of J.P. Morgan
could be materially affected.

     J.P. Morgan's opinions are based on economic, market and other conditions
as in effect on, and the information made available to J.P. Morgan as of, the
date of such opinion. Subsequent developments may affect the written opinion
dated February 11, 2000. J.P. Morgan expressed no opinion as to the price at
which the Equity Office common shares or Cornerstone common stock will trade at
any future time.

     In accordance with customary investment banking practice, J.P. Morgan
employed generally accepted valuation methods in reaching its opinion. The
following is a summary of the material financial analyses utilized by J.P.
Morgan in connection with providing its opinion.

     Pro Forma Merger Analysis.  J.P. Morgan analyzed the effect of the merger
on, among other things, the estimated First Call funds from operations per fully
diluted Equity Office common share, including EOP Partnership units, for the
years ended December 31, 2000 and 2001. In doing so, J.P. Morgan combined the
projected operating results for Equity Office and Cornerstone and assumed
specified savings in general and administrative expenses as well as additional
costs related to the transaction as per estimates provided by the management of
Equity Office. J.P. Morgan observed a total projected post-merger incremental
accretion of $0.01 and $0.04 to Equity Office's First Call 2000 and 2001 funds
from operations estimates of $2.80 and $3.03 per share, respectively. The
analysis assumed a transaction closing date of June 30, 2000. In calculating the
purchase price for Cornerstone common stock and Cornerstone Partnership units, a
$25.68125 share price for Equity Office common shares was assumed.

     J.P. Morgan also analyzed the effect of the merger on Equity Office's 2000
pro forma equity market capitalization, total market capitalization, leverage
ratios and distribution payout ratio. In this regard, J.P. Morgan noted that:

     - the pro forma equity market capitalization for Equity Office would be
       approximately $8.9 billion, assuming a share price of $25.68125, which
       equaled Equity Office's average closing share price for the ten business
       days from January 24, 2000 through February 4, 2000, and assuming
       347,044,557 Equity Office common shares and EOP Partnership units
       outstanding after completion of the merger, and a total post-merger pro
       forma market capitalization of approximately $18.7 billion; these figures
       do not include the 250,770 shares of Cornerstone common stock already
       owned by Equity Office before the merger;

     - Equity Office's debt to total market capitalization ratio would increase,
       upon completion of the merger, from 43.8% before the merger to 48.9%
       after the assumption of Cornerstone's outstanding debt plus the
       incremental debt incurred for the cash portion of the transaction
       consideration and for the payment of transaction costs;

     - the ratio of debt plus preferred shares to total market capitalization
       also would increase from 48.2% to 52.2%, assuming the retirement of all
       outstanding Cornerstone preferred stock; and

                                       32
<PAGE>   45

     - Equity Office's intent is to retain its current distribution of $1.68 per
       share for the combined company.

     Net Asset Value Analysis.  Using Equity Office's projections of
Cornerstone's performance results for the twelve-month period beginning July 1,
2000 and projected asset and liability balances as of June 30, 2000, J.P. Morgan
calculated the net asset value per share for Cornerstone. In so doing, J.P.
Morgan applied a range of capitalization rates from 8.25% to 8.75% to Equity
Office's projections of Cornerstone's stabilized July 1, 2000 to June 30, 2001
net operating income for the properties, assuming no additional net operating
income from future acquisitions or developments. The resulting gross real estate
value was added to the gross value of Cornerstone's other assets, including
developments in progress, cash and marketable securities, and fee income (valued
at a 5.0x multiple of earnings before interest, taxes, depreciation and
amortization), less Cornerstone's outstanding debt, to arrive at an equity net
asset value. The equity net asset value per share was then calculated by
dividing the equity net asset value by the number of shares of Cornerstone
common stock and Cornerstone Partnership units outstanding on a fully diluted
basis. This analysis indicated a net asset value range of between $18.59 and
$20.35 per share. J.P. Morgan also noted that this range was generally
consistent with equity analysts' published net asset value per share estimates
for Cornerstone of $17.00 to $20.81, based on asset balances as of September 30,
1999.

     Discounted Cash Flow Analysis.  J.P. Morgan performed a discounted cash
flow analysis, that is, an analysis of the present value, calculated as of June
30, 2000, of the projected levered cash flows for the period from 2000 to 2004,
of Cornerstone based upon projections provided by Equity Office, using discount
rates reflecting an equity cost of capital ranging from 13.5% to 14.5% and
terminal value multiples applied to 2005 funds from operations ranging from 9.5x
to 10.5x. J.P. Morgan, for the purposes of this valuation, derived 2005 funds
from operations by growing 2004 funds from operations at a rate equal to the
compound annual growth rate of projected funds from operations from 2000-2004.
Based upon Equity Office's projections of Cornerstone cash flows for the years
2000 through 2004, the range of present values per share of Cornerstone common
stock was $18.27 to $20.35. J.P. Morgan noted that the implied price for
Cornerstone common stock was below the indicated discounted cash flow range.

     Selected Transaction Analysis.  Using publicly available information, J.P.
Morgan examined selected transactions with respect to purchase price per share
to calculate both implied transaction premia and implied transaction funds from
operations multiples. Specifically, J.P. Morgan reviewed all publicly-traded
REIT acquisitions since January 1, 1997 with total transaction values of at
least $500 million as follows:

<TABLE>
<CAPTION>
ACQUIROR                                                       TARGET
- --------                                                       ------
<S>                                             <C>
Olympus Real Estate Fund II L.P.                Walden Residential Properties, Inc.
Berkshire Realty Holdings, L.P.                 Berkshire Realty Company, Inc.
Duke Realty Investments, Inc.                   Weeks Corporation
The Irvine Company                              Irvine Apartment Communities, Inc.
ProLogis Trust                                  Meridian Industrial Trust, Inc.
Equity Residential Properties Trust             Merry Land & Investment Company Inc.
Security Capital Pacific Trust                  Security Capital Atlantic
                                                Incorporated
Equity Office Properties Trust                  Beacon Properties Corporation
Equity Residential Properties Trust             Evans Withycombe Residential, Inc.
Equity Residential Properties Trust             Wellsford Residential Property Trust
</TABLE>

     J.P. Morgan observed a range of implied transaction premia over the target
companies' share price one week before the announcement date of 5.4% to 29.8%
with a median of 20.4%, resulting in a range of equity values for Cornerstone's
common stock of between $15.68 and $19.31 per share. In addition, J.P. Morgan
observed a range of implied transaction First Call funds from operations
multiples for the target companies from 9.4x to 14.8x with a median of 12.4x.
This range was then applied to Cornerstone's First Call 2000 funds from
operations per share of $1.69, resulting in a range of equity values for
Cornerstone's common stock of between $15.92 and $24.94 per share. J.P. Morgan
noted that the implied

                                       33
<PAGE>   46

price for Cornerstone common stock was within the range based on both
transaction premia and transaction multiples.

     Public Trading Multiples Analysis.  Using publicly available information,
J.P. Morgan compared selected financial and stock market data of Cornerstone
with similar data for selected publicly-traded companies which J.P. Morgan
judged to be analogous to that of Cornerstone's. These companies generally were
those with sufficiently large total market capitalization and specialization in
the office REIT sector. The companies selected by J.P. Morgan were:

     - Boston Properties Inc.;

     - CarrAmerica Realty Corporation;

     - Equity Office Properties Trust;

     - Mack-Cali Realty Corporation;

     - Speiker Properties; and

     - Vornado Realty Trust.

     For each comparable company, publicly available financial performance data
through the nine months ended September 30, 1999 was measured. J.P. Morgan
calculated the multiples of current stock price, as of February 4, 2000, to
analysts' estimates for 2000 First Call funds from operations for each of the
comparable companies to determine the 2000 funds from operations trading
multiples. J.P. Morgan's calculations resulted in a range of 2000 funds from
operations multiples from 7.0x to 10.7x with a median of 9.1x. These multiples
were then applied to Cornerstone's First Call 2000 funds from operations per
share estimate as of February 4, 2000 of $1.69, yielding a range of implied
trading values for Cornerstone's common stock of approximately $11.83 to $18.08
per share. J.P. Morgan noted that the implied price for Cornerstone common stock
was within the indicated trading multiples range. J.P. Morgan also observed that
the comparable companies had a range of debt to total market capitalization of
38.8% to 56.7%.

     Share Trading History Analysis.  Based on publicly available information,
J.P. Morgan reviewed the history of trading prices for Cornerstone common stock
for the 52-week and 104-week periods ending February 4, 2000. The 52-week
trading range was $13.06 to $16.94 per share and the 104-week trading range was
$13.06 to $18.38 per share. J.P. Morgan noted that the implied price for
Cornerstone common stock was at a premium to the 52-week high, but at a discount
to the 104-week high.

     The summary set forth above is not a complete description of the analyses
or data presented by J.P. Morgan. The preparation of a fairness opinion is a
complex process and is not necessarily susceptible to partial analysis or
summary description. J.P. Morgan believes that the summary set forth above and
their analyses must be considered as a whole and that selecting portions,
without considering all of its analyses, could create an incomplete view of the
processes underlying its analyses and opinion. J.P. Morgan based its analyses on
assumptions that it deemed reasonable, including assumptions concerning general
business and economic conditions and industry-specific factors. The other
principal assumptions upon which J.P. Morgan based its analyses are set forth
above under the description of each such analysis. J.P. Morgan's analyses are
not necessarily indicative of actual values or actual future results that might
be achieved, which values may be higher or lower than those indicated. Moreover,
J.P. Morgan's analyses are not and do not purport to be appraisals or otherwise
reflective of the prices at which businesses actually could be bought or sold.

     As a part of its investment banking business, J.P. Morgan and its
affiliates are continually engaged in the valuation of businesses and their
securities in connection with mergers and acquisitions, investments for passive
and control purposes, negotiated underwritings, secondary distributions of
listed and unlisted securities, private placements, and valuations for estate,
corporate and other purposes. J.P. Morgan was selected to deliver an opinion to
Equity Office's board of trustees with respect to the merger on the basis of
such experience and its familiarity with Equity Office.

                                       34
<PAGE>   47

     For the delivery of its opinion and related advisory work, J.P. Morgan will
receive a total fee of $7.0 million from Equity Office. $1.0 million was due and
payable upon announcement of the merger, with the remaining $6.0 million payable
upon the successful close of the merger. In addition, Equity Office reimbursed
J.P. Morgan for its reasonable expenses incurred in connection with its
services, including the fees and disbursements of its counsel, and agreed to
indemnify J.P. Morgan against certain liabilities, including liabilities arising
under the federal securities laws.

     J.P. Morgan and its affiliates, including Morgan Guaranty Trust Company of
New York, maintain banking and other business relationships with Equity Office
and its affiliates pursuant to which J.P. Morgan has received an aggregate of
approximately $3.1 million in fees over the past two years. In July 1998, J.P.
Morgan provided a fairness opinion to the William Wilson & Associates investors
relating to the merger of WWA with Cornerstone. J.P. Morgan received a fee of
$500,000 from the WWA investors for the delivery of its opinion in the
transaction. In the ordinary course of their businesses, J.P. Morgan and its
affiliates may actively trade the debt and equity securities of Equity Office or
Cornerstone for their own accounts or for the accounts of customers and,
accordingly, they may at any time hold long or short positions in such
securities.

CORNERSTONE'S REASONS FOR THE MERGER; RECOMMENDATION OF THE CORNERSTONE BOARD

     The Cornerstone board of directors approved the merger and approved and
adopted the merger agreement at a meeting held on February 11, 2000. The
Cornerstone board believes that the terms of the merger, the merger agreement
and the other transactions contemplated thereby are advisable and in the best
interests of Cornerstone and its stockholders. Accordingly, the Cornerstone
board recommends that Cornerstone stockholders vote to approve the merger
agreement.

     In considering the recommendation of the Cornerstone board with respect to
the merger agreement, Cornerstone stockholders should be aware that, following
completion of the merger, three current members of the Cornerstone board will
become trustees of Equity Office. Also, Messrs. Wilson, Moody and Dimock will
receive severance benefits as a result of the merger. See "-- Interests of
Cornerstone's Directors, Officers and Significant Stockholders in the
Merger -- Retention Agreements" on page 43. Therefore, these Cornerstone
directors have interests in the merger that are different from, or in addition
to, the interests of stockholders of Cornerstone generally.

     In its deliberations with respect to the merger and the merger agreement,
the Cornerstone board consulted with Cornerstone's management and the financial
and legal advisors to Cornerstone. The factors considered by the Cornerstone
board include those described below. While all of these factors were considered
by the Cornerstone board, the board did not make determinations with respect to
each factor. Rather, the board made its judgment with respect to the merger and
the merger agreement based on the total mix of information available to it, and
the judgments of individual directors may have been influenced to a greater or
lesser degree by their individual views with respect to different factors.

Positive Factors Considered by the Cornerstone Board

     In making its determination with respect to the merger and the merger
agreement, the Cornerstone board considered the following material positive
factors:

     - Each share of Cornerstone common stock will be converted, at the election
       of the stockholder, into either $18.00 per share in cash, without
       interest, or 0.7009 of a common share of Equity Office, subject to
       proration if the cash election is oversubscribed or undersubscribed.
       Consequently, based on the closing price of Equity Office common shares
       of $25.6875 on February 10, 2000, the last full trading day before the
       merger was publicly announced, each Cornerstone common share will be
       exchanged in the merger for either $18.00 in cash or for Equity Office
       common shares having a value of approximately $18.00, representing a 21%
       premium over the closing price of $14.875 for Cornerstone common stock on
       February 10, 2000;

                                       35
<PAGE>   48

     - The combined company will have an increased presence in major markets,
       which should enhance revenue opportunities by providing additional space
       options to existing Cornerstone and Equity Office tenants;

     - The increased diversity of markets should reduce the combined company's
       dependence on any single market or region, thereby reducing the risk that
       a regional economic downturn could affect a large number of the combined
       company's properties;

     - Based on the average closing price of Equity Office's common shares for
       the ten business days from January 24, 2000 through February 4, 2000, the
       combined company would have a total market capitalization of
       approximately $18.7 billion. By virtue of its larger size and investment
       grade credit rating, the Cornerstone board believes that the combined
       company will have improved access to capital markets, which should make
       additional debt or other financing available upon more attractive terms.
       The Cornerstone board also viewed as favorable its belief, based in part
       on discussions with financial advisors, that the increased market
       capitalization would provide Cornerstone stockholders with enhanced
       liquidity following consummation of the merger;

     - The Cornerstone board viewed as favorable the fact that the stock
       consideration to be issued in the merger will be tax-free for U.S.
       federal income tax purposes to U.S. Cornerstone stockholders who receive
       only stock;

     - The Cornerstone board believes Cornerstone and Equity Office have
       complimentary product focus and operating strategies, which should result
       in a successful integration of the two companies;

     - The Cornerstone board believes the merger may generate additional
       efficiencies through anticipated cost savings and reductions in expense;

     - The opinion, analysis and presentations of Lazard described under
       "Opinion of Cornerstone's Financial Advisor" below, including the opinion
       of Lazard to the effect that, as of the date of such opinion, and based
       upon and subject to the matters stated therein, the consideration to be
       received in the merger was fair, from a financial point of view, to the
       holders of common stock of Cornerstone; and

     - Cornerstone's other strategic alternatives, including the prospects of
       positioning Cornerstone for the future and enhancing long-term
       stockholder value by remaining an independent company or by effecting a
       strategic business combination with another company, were not as
       attractive as the current merger with Equity Office.

Negative Factors Considered by the Cornerstone Board

     The Cornerstone board of directors also considered the following
potentially negative factors in its deliberations concerning the merger and the
merger agreement:

     - The exchange ratio is fixed and not subject to adjustment. As a result, a
       decrease in the trading price of Equity Office common shares before the
       effective time of the merger will reduce the aggregate consideration
       received by the Cornerstone common stockholders;

     - The risk that the anticipated benefits of the merger to Cornerstone
       stockholders may not be realized as a result of possible changes in the
       real estate market in general, or as a result of the inability to achieve
       the anticipated reduction in expenses and other potential difficulties in
       integrating the two companies and their respective operations;

     - The effect of the public announcement of the merger on Cornerstone's
       ability to retain employees and on the trading price of Cornerstone's
       common stock;

     - The significant cost involved in connection with consummating the merger,
       the substantial management time and effort required to effectuate the
       merger and integrate the businesses of Cornerstone and Equity Office and
       the related disruption to Cornerstone's operations; and

                                       36
<PAGE>   49

     - The risk that the merger might not be completed based upon the failure to
       satisfy certain covenants or closing conditions.

     The Cornerstone board also considered certain matters described above under
"Risk Factors," including the potential benefits to certain directors and
officers discussed in "Risk Factors -- The directors and officers of Cornerstone
may have interests in the completion of the merger that are different from the
interests of Cornerstone's stockholders."

     In view of the wide variety of factors considered by the Cornerstone board,
the Cornerstone board did not quantify or otherwise attempt to assign relative
weights to the specific factors that it considered in making its determination.
In the view of the Cornerstone board, however, the potentially negative factors
considered by it were not sufficient, either individually or collectively, to
outweigh the positive factors considered in its deliberations relating to the
merger and the merger agreement.

OPINION OF CORNERSTONE'S FINANCIAL ADVISOR

     Opinion of Lazard.  At the meeting of the Cornerstone board of directors on
February 10, 2000, Lazard rendered its oral opinion to the Cornerstone board of
directors that, as of such date, the consideration to be received in the merger
was fair, from a financial point of view, to the holders of common stock of
Cornerstone. Lazard confirmed its February 10, 2000 oral opinion by delivering
to the Cornerstone board of directors its written opinion dated February 11,
2000, that, as of such date, the consideration to be received in the merger was
fair, from a financial point of view, to the holders of common stock of
Cornerstone. Lazard's analysis assumed that prior to the merger each holder of
units in Cornerstone Partnership will have elected to exercise its redemption
right and receive shares of common stock in Cornerstone in exchange therefor in
the manner provided in the merger agreement. Lazard has not been requested to
and will not update its opinion before the closing of the merger. No limitations
were imposed by the Cornerstone board of directors upon Lazard with respect to
the investigations made or the procedures followed by it in rendering its
opinion.

     THE FULL TEXT OF THE WRITTEN OPINION OF LAZARD DATED FEBRUARY 11, 2000,
WHICH SETS FORTH THE ASSUMPTIONS MADE, MATTERS CONSIDERED AND LIMITS ON THE
REVIEW UNDERTAKEN, IS ATTACHED AS ANNEX C TO THIS JOINT PROXY
STATEMENT/PROSPECTUS AND IS INCORPORATED HEREIN BY REFERENCE. CORNERSTONE
STOCKHOLDERS ARE URGED TO READ THE OPINION IN ITS ENTIRETY. LAZARD'S WRITTEN
OPINION, ADDRESSED TO THE CORNERSTONE BOARD OF DIRECTORS, IS DIRECTED ONLY TO
THE FAIRNESS OF THE CONSIDERATION TO BE RECEIVED IN THE MERGER TO THE HOLDERS OF
COMMON STOCK OF CORNERSTONE AND DOES NOT CONSTITUTE A RECOMMENDATION TO ANY
STOCKHOLDER OF CORNERSTONE AS TO HOW SUCH STOCKHOLDER OF CORNERSTONE SHOULD VOTE
AT THE CORNERSTONE SPECIAL MEETING. THE SUMMARY OF THE OPINION OF LAZARD SET
FORTH IN THIS JOINT PROXY STATEMENT/PROSPECTUS IS QUALIFIED IN ITS ENTIRETY BY
REFERENCE TO THE FULL TEXT OF SUCH OPINION.

     In arriving at its opinion, Lazard, among other things:

     - reviewed the merger agreement;

     - analyzed historical business and financial information relating to
       Cornerstone and Equity Office, including the audited financial statements
       for the twelve months ended December 31, 1998 and the unaudited financial
       statements for the nine months ended September 30, 1999 of Cornerstone
       and Equity Office;

     - reviewed the historical stock prices and trading volumes of Cornerstone's
       common stock and Equity Office's common shares;

     - reviewed financial forecasts and other data provided to Lazard by
       Cornerstone and Equity Office relating to their businesses;

     - reviewed public information with respect to certain other companies in
       lines of businesses Lazard believed to be comparable to the business of
       Cornerstone;

                                       37
<PAGE>   50

     - reviewed the financial terms of certain recent business combinations
       involving companies in lines of business Lazard believed to be comparable
       to those of Cornerstone;

     - held discussions with members of the senior management of Cornerstone and
       Equity Office with respect to the respective businesses of Cornerstone
       and Equity Office; and

     - conducted such other financial studies, analyses and investigations as
       Lazard deemed appropriate for the purposes of its opinion.

     Lazard relied upon and assumed, without independent verification, the
accuracy and completeness of all information that was furnished to it by Equity
Office and Cornerstone or otherwise reviewed by Lazard, and Lazard has not
assumed any responsibility or liability for the independent verification of such
information or any independent valuation or appraisal of any of the assets or
liabilities of Cornerstone or Equity Office, or concerning the solvency or fair
value of Cornerstone, Cornerstone Partnership, Equity Office, EOP Partnership or
any other entity. In relying on financial analyses and forecasts provided to it,
Lazard has assumed that they have been reasonably prepared based on assumptions
reflecting the best currently available estimates and judgments by management as
to the expected future results of operations and financial condition of Equity
Office and Cornerstone to which such analyses or forecasts relate. Lazard
assumed no responsibility for and expressed no view as to such forecasts or the
assumptions on which they were based. Lazard has also assumed that the merger
will have the tax consequences described in discussions with representatives of
Cornerstone.

     No representation or warranty was made by any party with respect to the
financial analyses and estimates referred to above. Financial projections are
subject to contingencies beyond management's control and realization of the
projections depends on numerous factors, including among other things, the cost
of integrating the companies, the completion of pending developments, the actual
cost in relation to such projects and decisions by management to modify business
plans to address changing needs and a changing operating environment. All
material events and circumstances cannot be predicted and unanticipated events
and circumstances are likely to occur. Accordingly, there may be differences
between the projected results of operations and the actual results of operations
of the respective companies, and such differences could be material. In the
event that the financial projections prove to be materially different, the
conclusions reached in the opinion of Lazard could be materially affected.

     In rendering its opinion, Lazard assumed that the merger would be
consummated on the terms described in the merger agreement, without any waiver
of any material terms or conditions by Cornerstone or Cornerstone Partnership,
and that obtaining the necessary regulatory approvals for the merger would not
have a material adverse effect on Cornerstone or Cornerstone Partnership.
Lazard's opinions are based on economic, market and other conditions as in
effect on, and the information made available to Lazard as of, the date of such
opinion. Subsequent developments may affect the written opinion dated February
11, 2000. Lazard expressed no opinion as to the price at which the Equity Office
common shares or Cornerstone common stock will trade at any future time.

     In arriving at its opinion and making its presentation to the Cornerstone
board of directors at the February 10, 2000 meeting of the board, Lazard
considered and discussed certain financial analyses and other factors. In
connection with its presentation on February 10, 2000, Lazard provided the
Cornerstone board of directors with a summary of results obtained by using
several different valuation methods as well as other materials concerning the
common stock and business of Cornerstone.

     The following paragraphs summarize Lazard's presentation.

     Public Market Valuation Analysis.  Lazard reviewed and compared the
financial and market performance of the following group of eight public United
States REITs:

     - Spieker Properties;

     - Boston Properties;

     - Equity Office;

                                       38
<PAGE>   51

     - Vornado Realty;

     - Duke-Weeks Realty;

     - CarrAmerica Realty;

     - Mack-Cali Realty; and

     - Crescent Real Estate.

     Lazard examined certain publicly available financial data of the selected
companies and published consensus funds from operations estimates for the
selected companies to derive multiples of share price to projected funds from
operations per share for the fiscal years ending December 31, 1999 and December
31, 2000. The merger price implied funds from operations multiples of 11.39x and
10.65x for 1999 and 2000, respectively, which are higher than the median funds
from operations multiples of the selected companies of 9.49x and 8.56x for 1999
and 2000, respectively.

     Comparable Transaction Analysis.  Lazard reviewed the consideration
proposed to be paid in 18 other comparable acquisitions of publicly-traded REIT
companies. Specifically, Lazard reviewed the following transactions:

<TABLE>
<CAPTION>
ACQUIROR                                                     TARGET                       DATE
- --------                                                     ------                     --------
<S>                                         <C>                                         <C>
Equity Office Properties Trust              Beacon Properties Corp.                     1997
Excel Realty Trust Inc.                     New Plan Realty Trust                       1998
Equity Residential Properties Trust         Evans Withycombe Residential                1997
Irvine Co.                                  Irvine Apartment Communities                1998
Kimco Realty Corp.                          Price REIT Inc.                             1998
Equity Residential Properties Trust         Wellsford Residential Property              1997
Bay Apartment Communities Inc.              Avalon Properties Inc.                      1998
Security Capital Pacific Trust              Security Capital Atlantic Inc.              1998
Post Properties, Inc.                       Columbus Realty Trust                       1997
Prologis Trust                              Meridian Industrial Trust Inc.              1998
Equity Residential Properties Trust         Merry Land & Investment Co. Inc.            1998
Apartment Investment & Management Co.       Ambassador Apartments Inc.                  1997
Camden Property Trust                       Oasis Residential Inc.                      1997
Duke Realty Investments Inc.                Weeks Corporation                           1999
Aptco LLC                                   Berkshire Realty Co. Inc.                   1999
Public Storage Inc.                         Storage Trust Realty                        1998
Reckson Associates                          Tower Realty Trust Inc.                     1998
Prime Retail Inc.                           Horizon Group Inc.                          1997
</TABLE>

For each such transaction, Lazard calculated the multiples of merger price to
projected funds from operations per share. The funds from operations multiple
implied by the merger price was 10.54x, which was below 11.63x, the median
implied funds from operations multiple for the selected transactions. However,
Lazard did not view such result as significant due to the substantial decrease
in the stock prices and funds from operations multiples of office and other
REITs since August 1998. Comparing only those mergers announced since August 1,
1998, the merger price is above the median implied funds from operations
multiple of 9.95x for such mergers.

     Discounted Cash Flow Analysis.  Lazard aggregated (x) the present value of
the projected cash flow of Cornerstone after interest expense and capital
expenditures over the three-year period from 2000 to 2002 with (y) the present
value of the range of terminal values described below. The range of terminal
values was calculated by applying multiples of 8.0x to 10.0x to Cornerstone's
projected funds from operations for the fiscal year 2003. This range of terminal
values represented Cornerstone's value beyond 2002. As part of the discounted
cash flow analysis, Lazard used discount rates ranging from 13% to 15%.

                                       39
<PAGE>   52

Based on the discounted cash flow analysis, Lazard calculated a range of the
equity value per share of Cornerstone of $14.37 to $18.01.

     Net Asset Value Analysis.  Lazard also calculated the combined company's
net asset value based on the net asset value for each company separately based
on an 8.25% capitalization rate on 2000 net operating income and other
information provided by each company's management. Certain adjustments were made
to arrive at the combined company's net asset value. The proposed transaction
provides a net increase to net asset value per share of $0.06 to Cornerstone
stockholders.

     Accretion/Dilution Analysis.  Lazard's merger model assumed that the merger
closes on July 1, 2000 and is based on modified versions of each company's
internal budget that excludes some investments not yet completed, including
selected development projects and acquisitions. General and administrative
expense reduction for the combined entity were assumed to save $20 million
annually. Purchase accounting was assumed for accounting purposes and debt was
marked-to-market and lease revenues were re-straight lined accordingly. Based on
the above assumptions and such budgets, Cornerstone's funds from operations per
share for 2001 was calculated to be $2.11 if the merger is completed versus
$1.79 if the merger is not completed.

     Premiums Paid Analysis.  Lazard reviewed the consideration proposed to be
paid in 18 other comparable acquisitions of publicly-traded REIT companies.
Specifically, Lazard reviewed the following transactions:

<TABLE>
<CAPTION>
ACQUIROR                                                     TARGET                     DATE
- --------                                                     ------                     ----
<S>                                         <C>                                         <C>
Equity Office Properties Trust              Beacon Properties Corp.                     1997
Aptco LLC                                   Berkshire Realty Co. Inc.                   1999
Irvine Co.                                  Irvine Apartment Communities                1998
Security Capital Pacific Trust              Security Capital Atlantic Inc.              1998
Equity Residential Properties Trust         Evans Withycombe Residential                1997
Kimco Realty Corp                           Price REIT Inc.                             1998
Excel Realty Trust Inc.                     New Plan Realty Trust                       1998
Prologis Trust                              Meridian Industrial Trust Inc.              1998
Equity Residential Properties Trust         Merry Land & Investment Co. Inc.            1998
Duke Realty Investments Inc.                Weeks Corporation                           1999
Camden Property Trust                       Oasis Residential Inc.                      1997
Reckson Associates                          Tower Realty Trust Inc.                     1998
Equity Residential Properties Trust         Wellsford Residential Property              1997
Apartment Investment & Management Co.       Ambassador Apartments Inc.                  1997
Post Properties, Inc.                       Columbus Realty Trust                       1997
Prime Retail Inc.                           Horizon Group Inc.                          1997
Bay Apartment Communities Inc.              Avalon Properties Inc.                      1998
Public Storage Inc.                         Storage Trust Realty                        1998
</TABLE>

For each such transaction, Lazard calculated the percentage premium represented
by the excess of (x) the acquisition price per share payable in such transaction
over (y) the closing stock price of the target company on the trading day
immediately prior to the announcement of such transaction. The implied premium
represented by the merger price of 21.52% was significantly higher than the
median of the implied premiums for the selected transactions of 11.71%.

     Stock Trading History.  Lazard examined the history of trading prices for
Cornerstone common stock. Cornerstone went public in April 1997 at a price of
$14.50 per share. Since its initial public offering, Cornerstone common stock
has traded as high as $20.00 per share and as low as $13.00 per share. Over the
52 weeks ended February 9, 2000, Cornerstone common stock traded as high as
$17.00 and as low as $13.00. The average closing price of Cornerstone common
stock during the 30-day period ended

                                       40
<PAGE>   53

February 9, 2000, was $14.97. The merger price represents a premium of 20.3%
over such 30-day average closing price and is 5.9% above Cornerstone's 52-week
high.

     The summary set forth above does not purport to be a complete description
of the analyses performed by Lazard. The preparation of a fairness opinion is a
complex process and is not necessarily susceptible to partial analysis or
summary description. In arriving at its opinion and in preparing its
presentation, Lazard performed a variety of financial analyses, the material
portions of which are summarized above. In addition, Lazard believes that its
analyses must be considered as a whole and that selecting portions of such
analyses and the factors considered by it, without considering all of such
analyses and factors, could create an incomplete view of the process underlying
its analyses set forth in the opinion and its presentation to the Cornerstone
board of directors. With regard to the public market valuation, premiums paid
and comparable transaction analysis summarized above, Lazard selected comparable
public companies on the basis of various factors, including the size of the
public company or portfolio of properties and similarity of the line of
business; however, no public company or portfolio of properties utilized as a
comparison is identical to Cornerstone. Accordingly, an analysis of the
foregoing is not mathematical; rather, it involves complex considerations and
judgments concerning differences in financial and operating characteristics of
the comparable companies or portfolios and other factors that could affect the
acquisition or public trading value of the comparable companies or portfolios to
which Cornerstone or its properties are being compared.

     In performing its analyses, Lazard made numerous assumptions with respect
to industry performance, general business and economic conditions and other
matters, many of which are beyond Cornerstone's control. Any estimates contained
in such analyses are not necessarily indicative of actual past or future results
or values, which may be significantly more or less than such estimates.
Estimates of values of companies or parts of companies do not purport to be
appraisals or necessarily to reflect the price at which such companies or parts
may actually be sold, and such estimates are inherently subject to uncertainty.

     Lazard is an internationally recognized investment banking firm that
regularly engages in the valuation of businesses and their securities in
connection with mergers and acquisitions. The Cornerstone board of directors
selected Lazard to act as its financial advisor on the basis of Lazard's
international reputation and experience in connection with the office REIT
industry, Cornerstone's prior relationship with Lazard and Lazard's familiarity
with Cornerstone.

     Under a letter agreement dated January 28, 2000 between Cornerstone and
Lazard, Lazard agreed to act as financial advisor to Cornerstone in connection
with the merger. Pursuant to the engagement letter, Cornerstone is obligated to
pay Lazard a fee of $1 million upon the execution of the definitive merger
agreement and upon the closing of the merger will be obligated to pay Lazard a
fee of $7 million. Cornerstone also agreed to reimburse Lazard for its
reasonable out-of-pocket expenses and to indemnify Lazard and certain related
persons against certain liabilities, including liabilities under the federal
securities laws, relating to, or arising out of, its engagement.

     Lazard and its affiliates maintain business relationships with Cornerstone
and its affiliates pursuant to which Lazard has received an aggregate of
approximately $9 million in fees since February 1, 1998. Lazard has not received
any fees over the past two years from Equity Office or its affiliates. In the
ordinary course of their businesses, Lazard and its affiliates may actively
trade the debt and equity securities of Equity Office or Cornerstone for their
own accounts or for the accounts of customers and, accordingly, they may at any
time hold long or short positions in such securities.

     As noted under the caption "Cornerstone's Reasons for the Merger;
Recommendation of the Cornerstone Board," the fairness opinion of Lazard was
only one of the many factors considered by the Cornerstone board of directors in
determining to approve and adopt the merger agreement.

EQUITY OFFICE BOARD OF TRUSTEES AND EXECUTIVE OFFICERS AFTER THE MERGER

     Following the merger, the current trustees of Equity Office will remain as
trustees of the combined entity. In addition, three current directors of
Cornerstone, John S. Moody, Jan H.W.R. van der Vlist and

                                       41
<PAGE>   54

William Wilson III, will become trustees of the combined entity, with their
terms expiring in 2002, 2003 and 2003, respectively. Under the terms of the
merger agreement, Equity Office is obligated to nominate Mr. van der Vlist for
reelection to the board of trustees in 2003 and 2006 if PGGM and its affiliates
continue to own at least 21,000,000 or more of the issued and outstanding Equity
Office common shares, as adjusted for stock splits or similar actions, at all
times up to the time the trustees are being elected in those years. If Mr. van
der Vlist fails to stand for re-election in 2003 or 2006 for any reason, or in
the event of his death or earlier resignation, and if PGGM continues to own at
least 21,000,000 or more of the issued and outstanding Equity Office shares as
described above, Equity Office is required take all action necessary to nominate
a replacement designated by PGGM for election or re-election as a trustee for an
additional three-year term, subject to Equity Office's approval if the
replacement is not an officer, director or employee of PGGM.

     Following the merger, the current executive officers of Equity Office will
remain as executive officers of Equity Office. No current executive officers of
Cornerstone will become executive officers of Equity Office following the
merger.

INTERESTS OF CORNERSTONE'S DIRECTORS, OFFICERS AND SIGNIFICANT STOCKHOLDERS IN
THE MERGER

     In considering the recommendation of the Cornerstone board with respect to
the merger, Cornerstone stockholders should be aware that, as described below,
certain Cornerstone directors and officers and PGGM have interests in the merger
that differ from, or are in addition to, the interests of stockholders
generally.

     Trustees of Equity Office After the Merger.  Following the merger, the
current trustees of Equity Office will remain as trustees of the combined
entity. In addition, three current directors of Cornerstone will become trustees
of the combined entity. See "-- Equity Office Board of Trustees and Executive
Officers after the Merger" on page 41.

     Equity-Based Awards.  Under Cornerstone's incentive stock plans, an
aggregate of 89,113 unvested shares of restricted common stock previously
awarded to executives who are parties to the retention agreements described
below will vest in full upon the merger and will be converted into cash or
common shares of Equity Office according to the terms of the merger agreement.
In addition, unvested options to purchase an aggregate of 3,177,476 shares of
Cornerstone common stock previously awarded to such executives will vest in full
upon the merger, and each option may be surrendered by the holder of the option
to Equity Office for cash equal to the difference between $18.00 per share and
the exercise price of the option or exchanged for options to purchase common
shares of Equity Office according to the terms of the merger agreement.
Cornerstone and Equity Office have agreed that the merger will constitute a
change in control for purposes of all applicable Cornerstone benefit and
compensation plans, including the Cornerstone 1998 Long Term Incentive Plan and
all stock option and restricted stock agreements provided under the terms of
such plans.

     Mr. Moody has agreed with Cornerstone to waive the acceleration of his
unvested restricted stock and unvested stock options.

     Moody Agreement.  Cornerstone has entered into a contract with Mr. Moody
whereby amounts are accrued under an unfunded arrangement to pay Mr. Moody a
supplemental pension. Under the contract, his supplemental pension account was
established with a credit of $250,000 as of July 1, 1995, and Cornerstone is
obligated to credit the account in the amount of $60,000 each subsequent July 1
during the continuance of Mr. Moody's employment.

     The account is also credited with any deemed income, gains or losses which
would be attributable to a corresponding investment of an equal cash amount in
such investment as Cornerstone, taking into account Mr. Moody's views, shall
deem the account to be invested. Payment of Mr. Moody's supplemental pension
account is to be made in a lump sum as of the date Mr. Moody terminates his
employment. However, if his employment is terminated for cause, Mr. Moody will
forfeit the account and no payment will be made. The contract also provides
that, outside of a change in control context, if Mr. Moody's

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<PAGE>   55

employment terminates for a reason other than cause, an additional credit of
$60,000 (or less, if Mr. Moody resigns without good reason) will be made to the
account. Following a change in control, in the event Mr. Moody's employment is
terminated by Cornerstone other than for cause, or if he resigns for good
reason, Cornerstone is obligated to credit his supplemental pension account with
an amount equal to $60,000 times the number of years, including fractional
portions of a year, remaining between his age on the date his employment ceases
and age 60.

     Retention Agreements.  Cornerstone has entered into retention agreements
with some of its officers to address the terms and conditions of their
employment in the event of a change in control. A "change in control" is
generally said to occur when: (a) any person becomes the owner of 25% or more of
Cornerstone's voting securities; (b) directors who constitute the Cornerstone
board at the beginning of any two-year period, and any new directors whose
election or nomination for election was approved by a vote of at least three
quarters of the directors then in office who either were directors at the
beginning of such period or whose election or nomination for election was
previously so approved, cease to constitute a majority; (c) a merger,
consolidation or reorganization in which Cornerstone's voting securities do not
continue to represent at least 50% of the surviving entity; or (d) a
reorganization, liquidation, or sale of all or substantially all of
Cornerstone's assets.

     In the event of termination of employment by Cornerstone without cause, or
by the employee for good reason, within two years following a change in control,
Cornerstone will make a lump sum payment of: (a) the pro rata portion of the
annual bonus for the year termination occurred, calculated on the basis of the
target bonus and on the assumption that all performance goals have been or will
be achieved; and (b) three times annual compensation in the case of Messrs.
Wilson, Moody, Van Boven and Dimock and two times annual compensation in the
case of all other officers. Annual compensation is generally defined as the
salary in effect immediately before termination or change in control, plus the
highest of the target annual bonus for the termination year or the average of
the three annual bonuses paid immediately prior to the termination or the change
in control.

     An officer may be terminated for "cause" if he or she is convicted of a
felony, reveals confidential information about Cornerstone or refuses to
substantially perform his duties. An officer can terminate for "good reason" if
Cornerstone reduces his salary or bonus target, reduces the value of employee
benefits materially, relocates his office more then 25 miles from the present
location, detrimentally alters his title or position, fails to get a successor
to agree to assume the obligations under the retention agreement or materially
breaches any provision of the agreement.

     In addition, the officer and his eligible dependents will continue to be
eligible to participate during the benefit continuation period in the medical,
dental, health, life and other welfare benefit plans and arrangements applicable
to him immediately before his or her termination of employment, on the same
terms and conditions in effect immediately before such termination. "Benefit
continuation period" means the period beginning on the day of termination and
ending on the earlier to occur of: (a) the third anniversary of the date of
termination in the case of Messrs. Wilson, Moody, Van Boven and Dimock and the
second anniversary of the date of termination in the case of all other officers;
and (b) the date that the officer is eligible and elects coverage under the
plans of a subsequent employer which provide substantially equivalent or greater
benefits to the officer and his dependents.

     If the payments to an officer under the retention agreement constitute
"parachute payments," as defined in Section 280G of the Internal Revenue Code,
and the officer would receive more value after taxes if the payments to be made
to the officer were capped at three times the "base amount" less $1.00, then the
payments will be reduced to that amount.

     Equity Office has agreed to assume and to perform Cornerstone's obligations
under the retention agreements in the same manner and to the same extent that
Cornerstone would be required to perform such obligations if the merger had not
taken place. The parties have agreed that the merger will be a "change in
control" for purposes of the retention agreements.

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<PAGE>   56

     Mr. Moody has agreed to waive any rights to payment of the lump sum payment
and to the continuation of benefits under his retention agreement in
consideration of Cornerstone's payment to Mr. Moody upon completion of the
merger, in recognition of his past service to Cornerstone, of $7,342,614 or, if
the merger is completed on or after July 1, 2000, $6,772,262. Messrs. Van Boven
and Dimock have each agreed to waive the payment of $1,000,000, and Mr. Moran
has agreed to waive the payment of $500,000, of the lump sum payment otherwise
payable to each of them in the event of the termination of their employment
other than for cause after the merger.

     Noncompetition Agreements.  Each of Messrs. Moody, Van Boven, Dimock and
Moran have agreed to enter into noncompetition agreements with Equity Office
that will restrict their ability to engage in business activities that are
competitive with those of Equity Office after the merger. In order to induce
Messrs. Moody, Van Boven, Dimock and Moran to enter into such agreements, Equity
Office will pay $5,000,000 to Mr. Moody, $1,000,000 to each of Messrs. Van Boven
and Dimock and $500,000 to Mr. Moran upon the completion of the merger.

     Escrowed Units.  In connection with the acquisition of William Wilson &
Associates in December 1998, an aggregate of 387,137 partnership units issued by
the Cornerstone Partnership that would have been distributable to the general
partners or managers of the four Wilson-sponsored opportunity funds acquired by
Cornerstone in the transaction were placed in escrow pursuant to an agreement
with certain investors in the opportunity funds. These units would have been
distributed to these general partners and managers under the governing
instruments of the opportunity funds if the units were valued at $17.25 per
unit, the agreed-upon value of the Cornerstone shares and units issued in the
Wilson transaction. The units were placed in escrow pending a determination of
their value during the one-year period beginning on the date the units were
released from the lockup provisions applicable to the units issued in the Wilson
acquisition. Mr. Wilson and other executive officers of Cornerstone indirectly
beneficially own in the aggregate 319,369 of such escrowed units.

THE PARTNERSHIP MERGER

     General.  In the partnership merger, Cornerstone Partnership will merge
into EOP Partnership and Cornerstone Partnership will cease to exist. Holders of
class A units of limited partnership interest in Cornerstone Partnership,
including Cornerstone, will receive, for each class A common unit issued and
outstanding immediately before the partnership merger, 0.7009 of a class A unit
of limited partnership interest in EOP Partnership.

     The partnership merger will be completed immediately before the merger.

     Redemption of Units; Participation in Cash/Share Election.  The merger
agreement provides that, before the partnership merger, holders of Cornerstone
Partnership units will have the right to have all or any portion of their
Cornerstone Partnership units redeemed by Cornerstone for shares of Cornerstone
common stock, which would enable limited partners of Cornerstone Partnership,
other than Cornerstone, to participate in the merger as stockholders of
Cornerstone. These redemptions will be conditioned upon the completion of the
partnership merger. In addition, limited partners of Cornerstone Partnership who
exercise this redemption right will be able to condition their redemption of
units upon the ability to receive in the merger only cash in exchange for all or
any specified portion of the Cornerstone common stock that would be issued in
the redemption. A separate consent solicitation/prospectus is being sent to each
limited partner of Cornerstone Partnership in connection with the partnership
merger.

     The merger agreement also provides that the units of limited partnership
interest issued in the partnership merger will have the right beginning
immediately after the partnership merger to redeem these units under the EOP
Partnership agreement. Upon redemption of these units, these limited partners
will receive Equity Office common shares on a one-for-one basis or their cash
equivalent, at the election of Equity Office, on the same basis as other limited
partners of EOP Partnership.

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<PAGE>   57

MERGER FINANCING

     Equity Office intends to finance the $1.1 billion cash portion of the
purchase price to Cornerstone common stockholders by obtaining lines of credit,
issuing unsecured notes and converting its existing line of credit to a term
loan. On March 21, 2000, EOP Partnership issued $500 million in unsecured notes
due March 2006. The notes have a fixed coupon rate of 8.4% per annum and an
effective rate of 8.5% per annum. EOP Partnership received net proceeds of
approximately $496.2 million, which were used to paydown its line of credit and
secured mortgage financing. After giving effect to the paydown of the line of
credit and secured debt, EOP Partnership had unused borrowing capacity under its
existing line of credit of approximately $776.5 million.

MATERIAL U.S. FEDERAL INCOME TAX CONSEQUENCES RELATING TO THE MERGER

     The following discussion summarizes the material U.S. federal income tax
consequences relating to the merger and the receipt of cash and/or Equity Office
common shares in the merger by holders of Cornerstone stock. Because this is a
summary that is intended to address only federal income tax consequences of the
merger that will apply to all Cornerstone stockholders, it may not contain all
the information that may be important to you. As you review this discussion, you
should keep in mind that:

     - the tax consequences to you may vary depending on your particular tax
       situation;

     - you may be subject to special rules that are not discussed below if you
       are:

       -- a tax-exempt organization;

       -- a broker-dealer;

       -- a non-U.S. person;

       -- a trust;

       -- an estate;

       -- a regulated investment company;

       -- an insurance company; or

       -- otherwise subject to special tax treatment under the Internal Revenue
          Code;

     - this summary does not address state, local, or foreign tax
       considerations;

     - this summary does not address all aspects of taxation that may be
       relevant to persons who are not citizens or residents of the United
       States; and

     - this discussion is not intended to be, and should not be construed as,
       tax advice.

YOU ARE URGED BOTH TO REVIEW THE FOLLOWING DISCUSSION AND TO CONSULT WITH YOUR
OWN TAX ADVISOR TO DETERMINE THE EFFECT OF THE MERGER ON YOUR INDIVIDUAL TAX
SITUATION, INCLUDING ANY STATE, LOCAL OR NON-U.S. TAX CONSEQUENCES.

     The information in this section is based on the current Internal Revenue
Code, current, temporary and proposed regulations, the legislative history of
the Internal Revenue Code, current administrative interpretations and practices
of the Internal Revenue Service, including its practices and policies as
endorsed in private letter rulings, which are not binding on the Internal
Revenue Service, and existing court decisions. Future legislation, regulations,
administrative interpretations and court decisions could change current law or
adversely affect existing interpretations of current law. Any change could apply
retroactively. Neither Equity Office nor Cornerstone has requested, or plans to
request, any rulings from the Internal Revenue Service concerning the tax
treatment of the merger. It is possible that the Internal Revenue Service would
challenge the statements in this discussion, which do not bind the Internal
Revenue Service or the courts, and that a court would agree with the Internal
Revenue Service.

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<PAGE>   58

FEDERAL INCOME TAX CONSEQUENCES OF THE MERGER TO CORNERSTONE, CORNERSTONE
STOCKHOLDERS, EQUITY OFFICE AND EQUITY OFFICE SHAREHOLDERS

     General.  The merger is intended to qualify as a "reorganization" under
section 368(a) of the Internal Revenue Code. The income tax consequences
summarized below are based on the assumption that the merger will qualify as a
reorganization. The obligation of Equity Office and Cornerstone to consummate
the merger is conditioned upon Hogan & Hartson L.L.P., counsel to Equity Office,
and King & Spalding, counsel to Cornerstone, delivering opinions to Equity
Office and Cornerstone, respectively, that the merger will qualify as a
reorganization under the provisions of section 368(a) of the Internal Revenue
Code. The opinions of counsel will rely on customary representations made by
Equity Office and Cornerstone and applicable factual assumptions. If any of the
factual assumptions or representations relied upon in the opinions of counsel
are inaccurate, the opinions may not accurately describe the U.S. federal income
tax treatment of the merger, and this discussion may not accurately describe the
tax consequences of the merger.

     One particularly important representation that counsel will rely upon in
rendering their opinions will be that the aggregate fair market value of the
Equity Office common shares issued in the merger will represent at least 40% of
the aggregate value of the total consideration issued by Equity Office in the
merger, as of the closing of the merger and taking into account any cash paid
for fractional Equity Office common shares. For example, if the merger had
occurred on       , 2000, then based on the trading price of Equity Office
common shares on that date and the number of shares of Cornerstone common stock
issued and outstanding on that date, and assuming that no holders of Cornerstone
Partnership units had converted their units into Cornerstone common stock
immediately before the merger, the aggregate fair market value of the Equity
Office common shares that would have been issued in the merger would represent
approximately      % of the value of the total consideration paid by Equity
Office in the merger. There is no guarantee, however, that the trading price of
the Equity Office common shares will be high enough at the scheduled closing of
the merger to allow counsel to render the required tax opinions and the merger
to be consummated.

     Federal Income Tax Consequences of the Merger to Cornerstone Common
Stockholders.  If the merger of Cornerstone with and into Equity Office
constitutes a reorganization within the meaning of section 368(a) of the
Internal Revenue Code, the merger will have the following material federal
income tax consequences to Cornerstone common stockholders:

     - Equity Office Common Shares Only.  The exchange in the merger of
       Cornerstone common stock solely for Equity Office common shares will not
       result in the recognition of gain or loss to a Cornerstone stockholder,
       except with respect to cash received for a fractional Equity Office
       common share, as discussed below, and possibly for a non-U.S. holder. See
       "Special Federal Income Tax Consequences of the Merger to Non-U.S.
       Holders of Cornerstone Stock" described below.

            A Cornerstone common stockholder that receives only Equity Office
       common shares in the merger will have a tax basis in the Equity Office
       common shares received equal to the stockholder's adjusted tax basis in
       the Cornerstone common stock exchanged.

     - Equity Office Common Shares and Cash.  A Cornerstone common stockholder
       that receives both Equity Office common shares and cash, other than cash
       received for a fractional Equity Office common share, will recognize gain
       equal to the lesser of either the cash received or the amount by which
       (A) the cash plus the fair market value of the Equity Office common
       shares received exceeds (B) the stockholder's adjusted tax basis in its
       Cornerstone common stock. Special considerations apply to a non-U.S.
       holder. See "Special Federal Income Tax Consequences of the Merger to
       Non-U.S. Holders of Cornerstone Stock" described below. A Cornerstone
       common stockholder that receives both Equity Office common shares and
       cash in the merger will not recognize any loss on the exchange.

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<PAGE>   59

     In general, gain recognized by a Cornerstone common stockholder that
receives both cash and Equity Office common shares will be taxable as capital
gain, assuming that the Cornerstone common stock was held as a capital asset.
      However, it is possible that the gain recognized by a Cornerstone common
      stockholder that receives both cash and Equity Office common shares will
      be taxable as dividend income if the cash received does not result in a
      "meaningful reduction" in the Equity Office common shares that the
      Cornerstone common stockholder would have received had it received only
      Equity Office common shares in the merger. In determining whether a
      meaningful reduction has occurred, section 318 of the Internal Revenue
      Code requires that the stockholder be treated as actually owning Equity
      Office common shares that are owned by the stockholder's family members or
      by entities in which the stockholder owns an interest, or which the
      stockholder has an option to acquire.

     Gain required to be recognized due to the receipt of cash as part of the
merger consideration generally must be calculated separately for each block of
      Cornerstone common stock exchanged in the merger. A block of stock is
      generally considered to be a group of shares acquired at the same cost in
      a single transaction.

     A Cornerstone common stockholder that receives both Equity Office common
shares and cash, other than cash received for a fractional Equity Office common
      share, will have a tax basis in the Equity Office common shares received
      equal to the stockholder's tax basis in its Cornerstone common stock
      exchanged, decreased by the amount of any cash received and increased by
      the amount of gain recognized in the exchange.

     - Cash Only.  A Cornerstone stockholder that receives only cash in the
       merger generally will recognize capital gain or loss equal to the
       difference between the amount of cash received and the stockholder's
       adjusted tax basis in its Cornerstone common stock, assuming that the
       Cornerstone common stock was held as a capital asset.

     - Fractional Shares.  A Cornerstone common stockholder that receives cash
       instead of a fractional Equity Office common share will be treated as if
       the fractional share was received in the merger and then redeemed by
       Equity Office. The Cornerstone common stockholder generally will
       recognize capital gain or loss equal to the difference between the amount
       of cash received for the fractional share and the stockholder's tax basis
       in the fractional share, assuming that the Cornerstone common stock was
       held as a capital asset.

     - Holding Period.  The holding period of the Equity Office common shares
       received by a Cornerstone common stockholder in the merger will include
       the holding period of the Cornerstone common stock exchanged, assuming
       that the Cornerstone common stock was held as a capital asset.

     Federal Income Tax Consequences of the Merger to Cornerstone Preferred
Stockholders.  Under the merger agreement, holders of Cornerstone 7% cumulative
convertible preferred stock will receive $18.00 cash, without interest, together
with accrued and unpaid dividends up to the closing of the merger. The holders
of Cornerstone 7% cumulative convertible preferred stock will be treated as if
they had sold their shares for the cash consideration received in the merger. As
a result, a holder of Cornerstone 7% cumulative convertible preferred stock
generally will recognize capital gain or loss measured by the difference between
the amount of cash received and the stockholder's adjusted tax basis in the
Cornerstone 7% cumulative convertible preferred stock that is converted in the
merger, assuming the stock was held as a capital asset.

     Backup Withholding.  Backup withholding tax at a rate of 31% may apply to
cash paid in the merger to a Cornerstone stockholder. Backup withholding will
not apply, however, if the Cornerstone stockholder:

     - furnishes a correct taxpayer identification number and certifies that he
       or she is not subject to backup withholding on Internal Revenue Service
       Form W-9, or an appropriate substitute form;

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<PAGE>   60

     - provides a certificate of foreign status on Internal Revenue Service Form
       W-8 or W-8 BEN, or an appropriate substitute form; or

     - is otherwise exempt from backup withholding.

     The Internal Revenue Service may impose a penalty upon any taxpayer that
fails to provide the correct taxpayer identification number.

     Special Federal Income Tax Consequences of the Merger to Non-U.S. Holders
of Cornerstone Stock. A "non-U.S. holder" is any stockholder other than:

     - a citizen or resident of the United States;

     - a corporation, partnership, or other entity treated as a corporation or
       partnership for federal income tax purposes created or organized in or
       under the laws of the United States, or of any state or the District of
       Columbia, unless in the case of a partnership Treasury Regulations
       otherwise require;

     - an estate the income of which is subject to U.S. federal income taxation
       regardless of its source; or

     - a trust whose administration is subject to the primary supervision of a
       U.S. court and which has one or more U.S. persons who have the authority
       to control all substantial decisions of the trust.

     Generally, a non-U.S. holder is subject to taxation under the Foreign
Investment in Real Property Tax Act, or FIRPTA, when it disposes of a "U.S. real
property interest," subject to limited exceptions. In general, shares of a REIT
are U.S. real property interests unless:

     - the REIT is a "domestically-controlled REIT;" or

     - a publicly-traded exception applies.

Under FIRPTA, the nonrecognition provisions of the Internal Revenue Code that
otherwise apply to a reorganization will only apply to a Cornerstone stockholder
in the merger if both the Cornerstone common stock exchanged and the Equity
Office common shares received in the merger qualify as U.S. real property
interests.

     Generally, a REIT is a domestically-controlled REIT if, at the time of the
determination and throughout the preceding five years, less than 50% of the fair
market value of its outstanding stock is directly or indirectly held by non-U.S.
persons. Cornerstone does not qualify as a domestically-controlled REIT and,
accordingly, Cornerstone stock constitutes a U.S. real property interest, unless
the publicly-traded exception applies. Equity Office, on the other hand,
believes that it qualifies as a domestically-controlled REIT and, thus, its
shares do not constitute U.S. real property interests. Consequently, although
the merger will otherwise qualify as a reorganization, a non-U.S. Cornerstone
common stockholder will recognize any taxable gain under FIRPTA that is realized
on the disposition of the Cornerstone common stock, unless the publicly-traded
exception applies. The amount of taxable gain for this purpose would be the
difference between:

     - the amount of any cash and the fair market value of the Equity Office
       common shares received in the merger; and

     - the holder's adjusted tax basis in its Cornerstone common stock.

The gain generally would be subject to FIRPTA tax at the normal U.S. tax rates
on capital gains and would be taxable even though the merger otherwise qualifies
as a nonrecognition transaction under the Internal Revenue Code. However, Equity
Office will not be required to withhold tax on taxable gain attributable to the
Cornerstone common stock.

     The publicly-traded FIRPTA exception will apply to a Cornerstone common
stockholder if Cornerstone common stock is "regularly traded" on an "established
securities market," as those terms are defined in applicable Treasury
Regulations, at any time during the calendar year, and if the stockholder's
actual and constructive ownership of Cornerstone's common stock, taking into
account the constructive ownership rules in the Internal Revenue Code, does not
exceed 5% of the value of Cornerstone common
                                       48
<PAGE>   61

stock at any time during the five-year period preceding the merger. The trading
of Cornerstone's common stock on the New York Stock Exchange meets the
requirements of the Treasury Regulations. Thus, a non-U.S. Cornerstone common
stockholder who did not own, taking into account the constructive ownership
rules of the Internal Revenue Code, more than 5% of the value of Cornerstone's
outstanding common stock during the preceding five years will not be subject to
tax on the disposition of Cornerstone common stock in the merger, even if the
stockholder receives cash in the merger. However, a non-U.S. Cornerstone common
stockholder who satisfies the publicly-traded exception, but receives cash in
the merger would nevertheless be subject to U.S. tax either:

     - in the same manner as a U.S. stockholder if the non-U.S. stockholder's
       investment in Cornerstone common stock is "effectively connected" with a
       United States trade or business of the non-U.S. stockholder; or

     - at a 30% rate on any recognized net capital gain if the non-U.S.
       stockholder is a nonresident alien individual who is present in the
       United States for 183 days or more during the taxable year and has a tax
       home in the United States.

     A non-U.S. Cornerstone preferred stockholder will be subject to FIRPTA at
the time of the merger unless the preferred stock actually and constructively
owned by the stockholder, at the time it was acquired, did not have a fair
market value greater than the fair market value on that date of 5% of the total
outstanding Cornerstone common stock and the Cornerstone common stock was
publicly-traded at such time. Subsequent acquisitions of Cornerstone preferred
stock are aggregated with the initial purchase, and all such stock is valued as
of the most recent acquisition date, in applying the 5% test. Equity Office will
be required to withhold tax from a non-U.S. Cornerstone preferred stockholder in
an amount equal to 10% of the cash paid in the merger unless the non-U.S.
holder:

     - can show conclusively that the 5% exception is applicable and withholding
       does not apply or

     - can provide a withholding certificate from the Internal Revenue Service
       showing that a lower amount of withholding is permitted.

     Federal Income Tax Consequences to Cornerstone, Equity Office and Equity
Office Shareholders. Equity Office and its shareholders will not recognize any
gain as a result of the merger, whether or not the merger qualifies as a
reorganization under section 368(a) of the Internal Revenue Code.

     Cornerstone will not recognize any gain as a result of the merger if the
merger qualifies as a reorganization under section 368(a) of the Internal
Revenue Code and Cornerstone qualifies as a "real estate investment trust," or
"REIT," at the time of the merger.

     Federal Income Tax Consequences of the Merger if the Merger Did Not Qualify
as a Reorganization or Cornerstone Did Not Qualify as a REIT.  Although it is a
condition to consummation of the merger that Equity Office and Cornerstone
receive opinions of counsel that the merger will qualify as a reorganization for
federal income tax purposes, these opinions will not be binding upon the
Internal Revenue Service or the courts. If the merger did not qualify as a
reorganization for federal income tax purposes, the merger would be treated as a
taxable transaction. Accordingly, Cornerstone would recognize gain or loss equal
to the difference between:

     - the sum of

      -- the fair market value of the Equity Office common shares issued in the
         merger;

      -- the cash issued in the merger; and

      -- the outstanding debt of Cornerstone including Cornerstone's allocable
         share of the outstanding debt of Cornerstone Partnership, as determined
         for tax purposes; and

     - Cornerstone's adjusted tax basis in its various properties and assets,
       including its interest in Cornerstone Partnership.

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<PAGE>   62

In computing its taxable income for the taxable year ending on the date of the
merger, Cornerstone generally would be entitled to a dividends paid deduction
equal to the fair market value of the Equity Office common shares and cash,
including cash paid for fractional shares, issued in the merger.

     A Cornerstone stockholder would recognize gain equal to the difference
between:

     - the aggregate fair market value of the Equity Office common shares and
       the cash received; and

     - the stockholder's adjusted tax basis in its Cornerstone stock.

     To the extent that Cornerstone's gain, if any, were to exceed the fair
market value of the Equity Office common shares and cash issued in the merger,
the gain generally would be taxable to Cornerstone, and the liability for this
tax would transfer to Equity Office.

     As a condition to the merger, Equity Office will receive an opinion from
King & Spalding, counsel to Cornerstone, that Cornerstone qualifies as a REIT at
the time of the merger. This opinion, however, will not be binding on the
Internal Revenue Service or the courts. If Cornerstone did not qualify as a REIT
at the time of the merger, Cornerstone would be required to recognize gain with
respect to the built-in gain attributable to its assets. Built-in gain is the
amount by which an asset's fair market value exceeds its adjusted tax basis. As
the successor to Cornerstone in the merger, Equity Office would be liable for
any tax owed by Cornerstone as a result of the recognition of built-in gain.
However, under recently published Treasury Regulations, Equity Office would be
able to make an election that would allow Cornerstone to avoid recognizing its
built-in gain if Cornerstone did not qualify as a REIT at the time of the
merger. This election would require Equity Office to recognize the built-in gain
at the highest regular corporate rate if Equity Office were to dispose of any
asset with built-in gain that was acquired from Cornerstone during the 10 years
following the merger. Even though Equity Office will receive an opinion of King
& Spalding, counsel to Cornerstone, that Cornerstone qualifies for taxation as a
REIT at the time of the merger, Equity Office intends to make a protective
election of the type described above as a precautionary measure in order to
ensure that Cornerstone would not recognize gain at the time of the merger if
the Internal Revenue Service later were to determine that Cornerstone did not
qualify as a REIT at the time of the merger.

PRE-MERGER DIVIDEND

     Under the merger agreement, Cornerstone may declare a dividend to its
stockholders immediately before the merger equal to the minimum amount necessary
for Cornerstone to satisfy the REIT distribution requirements under section
857(a)(1) of the Internal Revenue Code and to avoid the payment of tax with
respect to any undistributed income for Cornerstone's short taxable year ending
at the time of the merger. Section 857(a)(1) requires a REIT to distribute to
its stockholders each taxable year an amount equal to 95% of its "REIT taxable
income." In addition, a REIT is required to pay tax on any income that it does
not distribute to its shareholders, even if it satisfies the 95% distribution
requirement. If Cornerstone pays a pre-merger dividend, Equity Office will
declare a dividend to its common shareholders in an amount per share equal to
the amount per share of the pre-merger dividend paid by Cornerstone, divided by
the exchange ratio provided in the merger agreement. If Cornerstone and Equity
Office declare dividends as described above, the record dates will be the day
before the closing of the merger. Any pre-merger dividends paid to Cornerstone
stockholders and Equity Office shareholders will not be part of the cash
consideration paid in the merger and will be taxed in the same manner as other
dividends paid by Cornerstone and Equity Office.

REIT QUALIFICATION

     Both Equity Office and Cornerstone have elected to be taxed as REITs under
sections 856 through 860 of the Internal Revenue Code. A REIT generally is not
subject to federal income tax on the income that it distributes to shareholders
if it distributes at least 95% of its REIT taxable income and it meets other
requirements. For taxable years starting after December 31, 2000, REITs will
have to distribute only 90% of their REIT taxable income to shareholders.

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<PAGE>   63

     As a condition to the merger, Hogan & Hartson L.L.P., counsel to Equity
Office, will deliver an opinion to Cornerstone that commencing with Equity
Office's taxable year ending December 31, 1997, Equity Office was organized and
has operated in conformity with the requirements for qualification as a REIT,
and that after consummation of the merger, Equity Office's proposed method of
operation will enable it to continue to qualify as a REIT.

     This opinion will rely upon customary representations made by Equity Office
about factual matters relating to the organization and operation of Equity
Office, EOP Partnership and its subsidiaries, including BeaMetFed, Inc. In
addition, this opinion will be based upon factual representations of Equity
Office concerning its business and properties as set forth in this joint proxy
statement/prospectus and the other documents incorporated by reference in this
joint proxy statement/prospectus. Finally, the portion of the Hogan & Hartson
L.L.P. opinion that addresses the qualification of Equity Office as a REIT
following the merger will be based in part upon the opinion of King & Spalding
relating to the qualification of Cornerstone as a REIT at the closing of the
merger and the representations made by Cornerstone in connection with the King &
Spalding opinion.

     Equity Office intends to continue to operate in a manner to qualify as a
REIT following the merger, but there is no guarantee that Equity Office will
qualify or remain qualified as a REIT. Qualification and taxation as a REIT
depend upon Equity Office's ability to meet, through actual annual operating
results, requirements relating to asset ownership, distribution levels and
diversity of share ownership, and the various REIT qualification requirements
imposed under the Internal Revenue Code. Hogan & Hartson L.L.P. will not review
Equity Office's compliance with these tests on a continuing basis. Given the
complex nature of the REIT qualification requirements, the ongoing importance of
factual determinations and the possibility of future changes in the
circumstances of Equity Office, Equity Office cannot guarantee that its actual
operating results for any particular tax year will satisfy the requirements for
taxation as a REIT under the Internal Revenue Code.

     If Cornerstone did not qualify as a REIT in any of its prior tax years,
Cornerstone will become liable for federal income tax on its income earned in
any year it did not qualify as a REIT. In addition, if Cornerstone did not
qualify as a REIT at the time of the merger, Equity Office could have to pay
additional taxes if it sold any of the Cornerstone properties within 10 years
following the merger. As a condition to the merger, King & Spalding, counsel to
Cornerstone, will provide an opinion to Equity Office that Cornerstone,
commencing with its taxable year ended December 31, 1997 through its short
taxable year ending at the closing of the merger, was organized and has operated
in conformity with the requirements for qualification as a REIT. This opinion
will rely upon customary representations made by Cornerstone about factual
matters relating to the organization and operation of Cornerstone, Cornerstone
Partnership and their affiliates. Cornerstone intends to continue to operate in
a manner to qualify or remain qualified as a REIT until the closing of the
merger. Qualification and taxation as a REIT depend upon Cornerstone's ability
to meet, through actual annual operating results, requirements relating to asset
ownership, distribution levels and diversity of share ownership, and the various
REIT qualification requirements imposed under the Internal Revenue Code. There
is no guarantee that Cornerstone's actual operating results for any of its
taxable years, including its short taxable year ending at the closing of the
merger, will have satisfied the requirements for taxation as a REIT under the
Internal Revenue Code.

     To qualify as a REIT, Equity Office cannot have at the end of any taxable
year any undistributed earnings and profits that are attributable to a non-REIT
taxable year. As a result of the merger, Equity Office will inherit any
undistributed non-REIT earnings and profits that would result from the failure
of Cornerstone to qualify as a REIT at any point. Equity Office will receive an
opinion from King & Spalding regarding Cornerstone's REIT qualification, which
will rely on Cornerstone's representations to King & Spalding that Cornerstone
does not have any non-REIT earnings and profits attributable to any taxable year
in which Cornerstone did not elect or qualify to be taxed as a REIT, and that
Cornerstone will take all steps to ensure that it will not have earnings and
profits attributable to non-REIT taxable years at the close of Cornerstone's
short taxable year ending at the time of the merger.

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<PAGE>   64

OTHER INFORMATION RELEVANT TO EQUITY OFFICE'S REIT QUALIFICATION

     In order to qualify as a REIT, Equity Office also must derive at least 95%
of its gross income from specified qualifying sources. Following the merger,
Equity Office intends to operate the former Cornerstone properties in a manner
so that they do not produce material amounts of non-qualifying income. After the
merger, Equity Office will own and operate a hotel property currently owned by
Cornerstone. Income from the operation of a hotel does not qualify for purposes
of the REIT income tests. Equity Office intends to transfer this property to one
of its non-controlled subsidiaries. Equity Office believes that the amount of
nonqualifying income generated from this hotel will be an insignificant amount
and will not affect Equity Office's ability to meet the 95% gross income test.

     In addition, in order to qualify as a REIT, Equity Office cannot own more
than 10% of the voting securities of any issuer, and it cannot have more than 5%
of its assets invested in the stock and securities of any one issuer. Equity
Office will acquire in the merger a new non-controlled subsidiary, WCP Services,
Inc., currently owned by Cornerstone. Equity Office will own all of the
non-voting stock and 1% of the voting stock of WCP Services, Inc., which
together represents 95% of the economic interest. The remaining voting stock
representing 5% of the economic interest will be acquired by Equity Office
Property Management Corp., another non-controlled subsidiary of Equity Office.
WCP Services, Inc. is taxable as a regular "C" corporation. Equity Office does
not believe that the value of the securities of WCP Services, Inc. will cause it
to violate the 5% asset test.

U.S. TAXATION OF SALES OF EQUITY OFFICE COMMON SHARES BY NON-U.S. HOLDERS.

     Gain recognized by a non-U.S. shareholder upon the sale or exchange of
Equity Office common shares generally would not be subject to United States
taxation unless:

     - the investment in the Equity Office common shares is effectively
       connected with the non-U.S. shareholder's United States trade or
       business, in which case the non-U.S. shareholder will be subject to the
       same treatment as domestic shareholders with respect to any gain;

     - the non-U.S. shareholder is a nonresident alien individual who is present
       in the United States for 183 days or more during the taxable year and has
       a tax home in the United States, in which case the nonresident alien
       individual will be subject to a 30% tax on the individual's net capital
       gains for the taxable year; or

     - the Equity Office shares constitute a U.S. real property interest within
       the meaning of FIRPTA, as described below.

     The Equity Office common shares will not constitute a United States real
property interest if Equity Office is a domestically-controlled REIT. Equity
Office will be a domestically-controlled REIT if, at all times during a
specified testing period, less than 50% in value of its stock is held directly
or indirectly by non-U.S. shareholders.

     Equity Office believes that, currently, it is a domestically-controlled
REIT and, therefore, that the sale of Equity Office common shares would not be
subject to taxation under FIRPTA. Because the shares of Equity Office are
publicly traded, however, Equity Office cannot guarantee that it will continue
to be a domestically-controlled REIT.

     Even if Equity Office does not qualify as a domestically-controlled REIT at
the time a non-U.S. shareholder sells its Equity Office common shares, gain
arising from the sale still would not be subject to FIRPTA tax if:

     - the class or series of shares sold is considered regularly traded under
       applicable Treasury Regulations on an established securities market, such
       as the New York Stock Exchange, and

     - the selling non-U.S. shareholder owned, actually or constructively, 5% or
       less in value of the outstanding class or series of shares being sold
       throughout the five-year period ending on the date of the sale or
       exchange.

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<PAGE>   65

     If gain on the sale or exchange of Equity Office common shares were subject
to taxation under FIRPTA, the non-U.S. shareholder would be subject to regular
U.S. income tax with respect to any gain in the same manner as a taxable U.S.
shareholder, subject to any applicable alternative minimum tax and special
alternative minimum tax in the case of nonresident alien individuals.

AMENDMENTS TO EQUITY OFFICE'S DECLARATION OF TRUST RELATING TO
"DOMESTICALLY-CONTROLLED" REIT STATUS

     Based on the number of Equity Office common shares and shares of
Cornerstone common stock outstanding on the record date, PGGM could own up to
approximately      % of the Equity Office common shares following the merger, if
PGGM elects to convert its Cornerstone common stock into Equity Office common
shares in the merger. In order to assist Equity Office in continuing to qualify
as a domestically-controlled REIT following the merger and as required by the
merger agreement, amendments to the declaration of trust of Equity Office will
be effected as part of the merger. The text of these amendments is attached to
this joint proxy statement/prospectus as Annex D.

     The amendments restrict the direct or indirect acquisition and ownership of
Equity Office shares, if as a result of the acquisition or ownership, non-U.S.
persons would own directly or indirectly 43% or more of the fair market value of
the issued and outstanding Equity Office shares. This ownership restriction
would not apply to any acquisition of Equity Office preferred shares outstanding
at the time the amendments become effective, or Equity Office common shares
received upon the conversion of any Equity Office preferred shares. If any
transfers of Equity Office shares occur that would result in non-U.S. persons
owning directly or indirectly 43% or more of the fair market value of the issued
and outstanding Equity Office shares as described above, then the number of
shares that would cause a non-U.S. person to violate this restriction will be
automatically transferred to a charitable trust, or if transfer to a charitable
trust would not be effective to prevent violation of this restriction, then the
transfer of shares will be void.

     The board of trustees of Equity Office may increase the ownership
limitation to a percentage not to exceed 49% of the fair market value of the
issued and outstanding Equity Office shares, less the percentage of the
aggregate value of all outstanding shares attributable to the value of any
preferred shares that were issued and outstanding at the time the amendments
become effective or the common shares into which they are convertible. The
amendments also require each person who is a beneficial or constructive owner of
Equity Office shares to provide any information as Equity Office may require, in
good faith, in order to determine Equity Office's status as a
domestically-controlled REIT. There is no guarantee, however, that the
amendments will ensure that Equity Office continues to qualify as a
domestically-controlled REIT.

ACCOUNTING TREATMENT

     Equity Office will treat the merger as a purchase for financial accounting
purposes. This means that Equity Office will allocate the purchase price to
assets acquired and liabilities assumed from Cornerstone based on their
estimated fair values at the time the merger is completed. The amount of the
consideration paid to Cornerstone stockholders that exceeds the net fair value
of assets acquired and liabilities assumed will be allocated to investment in
real estate.

RESTRICTIONS ON RESALES BY AFFILIATES

     The Equity Office common shares to be issued to Cornerstone stockholders in
the merger will be freely transferable under the Securities Act, except for
shares issued to any person who may be deemed to be an "affiliate" of
Cornerstone within the meaning of Rule 145 under the Securities Act or who will
become an "affiliate" of Equity Office within the meaning of Rule 144 under the
Securities Act after the merger. Equity Office common shares received by persons
who are deemed to be Cornerstone affiliates or who become Equity Office
affiliates may be resold by these persons only in transactions permitted by the
limited resale provisions of Rule 145 or as otherwise permitted under the
Securities Act. Persons who may be deemed to be affiliates of Cornerstone
generally include individuals or entities that, directly or indirectly through
one or more intermediaries, control, are controlled by or are under common
control with Cornerstone and may include officers, directors and principal
stockholders of Cornerstone. All Cornerstone

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<PAGE>   66

stockholders who may be deemed to be affiliates of Cornerstone will be so
advised before the completion of the merger.

     Under the merger agreement, Cornerstone will use its commercially
reasonable efforts to obtain an affiliate agreement from each affiliate of
Cornerstone before the completion of the merger by which each Cornerstone
affiliate will agree not to sell, transfer, pledge or otherwise dispose of any
of the Equity Office common shares received in the merger in violation of the
Securities Act or the rules and regulations promulgated under the Securities
Act. Generally, this will require that all sales be made in accordance with Rule
145 under the Securities Act, which in turn requires that, for specified
periods, sales be made in compliance with the volume limitations, manner of sale
provisions and current information requirements of Rule 144 under the Securities
Act.

     Equity Office has the right to place legends on the certificates evidencing
Equity Office common shares issued to Cornerstone affiliates in the merger
summarizing the foregoing restrictions until a sale, transfer, pledge or other
disposition of the Equity Office common shares represented by these certificates
has been registered under the Securities Act or is made in compliance with Rule
145 under the Securities Act.

     Persons who are not affiliates of Cornerstone may generally sell their
Equity Office common shares without restrictions and without delivering this
joint proxy statement/prospectus.

NO DISSENTERS' RIGHTS

     Cornerstone is organized under Nevada law, and Equity Office is organized
under Maryland law. Under Nevada law, holders of Cornerstone common stock are
not entitled to dissenters' rights as a result of the merger. Under Maryland
law, holders of Equity Office common shares are not entitled to dissenters'
rights as a result of the merger. The holders of the Cornerstone preferred stock
have waived any dissenters' rights that they may have under Nevada law as a
result of the merger.

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<PAGE>   67

                              THE MERGER AGREEMENT

     The following is a summary of the material terms of the merger agreement
and is qualified in its entirety by reference to the merger agreement, which is
attached at the back of this joint proxy statement/prospectus as Annex A. You
should read the merger agreement as it is the legal document that governs the
merger.

CLOSING; EFFECTIVE TIME OF THE MERGER

     The completion of the merger will occur no later than the third business
day after the satisfaction or waiver of the conditions set forth in the merger
agreement or at such other date or time as may be agreed by Equity Office and
Cornerstone. If the merger is approved at the special meetings, Equity Office
and Cornerstone currently expect to complete the merger on that same day.

     As soon as practicable after all conditions to the completion of the merger
and the partnership merger are satisfied, and immediately after the completion
of the partnership merger, Equity Office and Cornerstone will execute and file
articles of merger with the State Department of Assessments and Taxation of
Maryland and with the Secretary of State of the State of Nevada. The effective
time of the merger will be the time specified in the articles of merger.

MERGER CONSIDERATION

     In the merger, holders of Cornerstone common stock may elect to receive
either $18.00 per share in cash, without interest, or 0.7009 of an Equity Office
common share, subject to proration if the cash election is oversubscribed or
undersubscribed, as described in "Proration of Cash and Equity Office Common
Shares Issued in the Merger" below.

     In the merger, holders of Cornerstone preferred stock will receive $18.00
per share in cash, without interest, together with accrued and unpaid dividends
through the date of the closing of the merger.

     Upon conversion of the outstanding shares of Cornerstone common stock and
preferred stock into merger consideration, the Cornerstone common stock and
preferred stock will be cancelled and retired and will cease to exist.

PRORATION OF CASH AND EQUITY OFFICE COMMON SHARES ISSUED IN THE MERGER

     A total of 58,551,525 shares of Cornerstone common stock will be converted
in the merger into the right to receive $18.00 per share in cash, without
interest, and the number of shares of Cornerstone common stock issued and
outstanding immediately before the merger, but after giving effect to
redemptions by holders of Class A common units of Cornerstone Partnership, in
excess of 58,551,525 shares will be converted into the right to receive Equity
Office common shares based on the 0.7009 exchange ratio.

     Because the aggregate cash portion of the merger consideration to be paid
to the Cornerstone common stockholders is fixed at approximately $1.054 billion,
or $18.00 times 58,551,525 shares, and because the number of Equity Office
common shares issuable in the merger will equal 0.7009 times the number of
shares of Cornerstone common stock issued and outstanding immediately before the
closing of the merger in excess of 58,551,525 shares, the amount of cash and the
number of Equity Office common shares that each Cornerstone stockholder may
receive in the merger may be subject to proration based on the elections made by
other Cornerstone common stockholders.

     For example:

     - if cash elections are received for a number of shares of Cornerstone
       common stock which is greater than 58,551,525 shares, then:

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<PAGE>   68

      -- each share of Cornerstone common stock for which an election is not
         received and each share of Cornerstone common stock for which a share
         election is received will be converted in the merger into 0.7009 of an
         Equity Office common share; and

      -- the shares of Cornerstone common stock for which cash elections are
         received will be converted in the merger into $18.00 in cash, without
         interest, and Equity Office common shares based on the 0.7009 exchange
         ratio, on a proportionate basis, so that the aggregate number of shares
         of Cornerstone common stock converted into cash in the merger equals
         58,551,525 shares, with Equity Office common shares issued for the
         remainder of the shares of Cornerstone common stock converted in the
         merger.

     - if share elections are received for a number of shares of Cornerstone
       common stock that is greater than (a) the number of shares of Cornerstone
       common stock issued and outstanding immediately before the merger less
       (b) the 58,551,525 shares to be converted into cash in the merger, then:

      -- each share of Cornerstone common stock for which an election is not
         received and each share of Cornerstone common stock for which a cash
         election is received will be converted in the merger into the right to
         receive $18.00 in cash, without interest; and

      -- the shares of Cornerstone common stock for which share elections are
         received will be converted in the merger into Equity Office common
         shares based on the 0.7009 exchange ratio, and $18.00 in cash, without
         interest, on a proportionate basis, so that the aggregate number of
         shares of Cornerstone common stock converted into cash in the merger
         equals 58,551,525 shares, with Equity Office common shares issued for
         the remainder of the shares of Cornerstone common stock converted in
         the merger.

     - if cash elections are received for less than the 58,551,525 shares of
       Cornerstone common stock to be converted in the merger into cash AND
       share elections are received for less than the maximum number of shares
       of Cornerstone common stock to be converted into Equity Office common
       shares in the merger, then:

      -- each share of Cornerstone common stock for which a cash election is
         received will be converted in the merger into the right to receive
         $18.00 in cash, without interest;

      -- each share of Cornerstone common stock for which a share election is
         received will be converted into Equity Office common shares based on
         the 0.7009 exchange ratio; and

      -- the shares of Cornerstone common stock for which no share elections are
         received will be converted into cash and Equity Office common shares so
         that the aggregate number of shares of Cornerstone common stock
         converted into $18.00 in cash, without interest, in the merger equals
         58,551,525 shares, with Equity Office common shares based on the 0.7009
         exchange ratio issued for the remainder of the shares of Cornerstone
         common stock converted in the merger.

     Because the exchange ratio is fixed at 0.7009, the market value of any
Equity Office common shares you receive in the merger may differ from $18.00 per
share and from the market value of the Equity Office common shares if the merger
occurred today. On             , 2000, the closing price of Equity Office common
shares was $     per share, which, based on the 0.7009 exchange ratio, would
equal a market value equivalent per share of Cornerstone common stock of
$          had the merger been completed on that date. You should obtain current
market prices for Equity Office common shares and shares of Cornerstone common
stock.

SURRENDER OF CORNERSTONE CERTIFICATES

     Equity Office has appointed EquiServe L.L.C. to act as exchange agent for
the purpose of paying the merger consideration. Equity Office will make
available to the exchange agent, as required under the merger agreement, the
cash and securities certificates for that purpose.

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<PAGE>   69

     Promptly after the closing of the merger, Equity Office will cause the
exchange agent to send each holder of Cornerstone common stock, other than such
holders of Cornerstone common stock who validly submitted a form of election and
their certificates for Cornerstone common stock to the exchange agent before the
Cornerstone special meeting as provided under "The Cornerstone Special
Meeting -- Voting and Election Procedures" on page 22, a letter of transmittal
for use in such exchange and instructions explaining how to surrender
certificates to the exchange agent. Holders of Cornerstone common stock whose
shares are converted into the right to receive the merger consideration and who
surrender their certificates to the exchange agent, together with a properly
completed and signed letter of transmittal, as the case may be, will receive the
merger consideration.

     Holders of unexchanged shares of Cornerstone common stock will not be
entitled to receive any dividends, interest payments or other distributions
payable by Equity Office on the Equity Office common shares. Upon surrender,
however, such holders will receive accumulated dividends and distributions,
without interest, payable on Equity Office shares subsequent to and in respect
of a record date after the closing of the merger, together with cash in lieu of
fractional shares.

TREATMENT OF CORNERSTONE STOCK OPTIONS

     Upon the completion of the merger, each Cornerstone stock option
outstanding under Cornerstone's 1998 long-term incentive plan or any other
arrangement, whether or not then vested or exercisable, other than options
surrendered as provided below, will become fully exercisable and vested and each
such Cornerstone stock option will be automatically converted upon the
completion of the merger into an option to purchase Equity Office common shares.
The substituted option will permit its holder to purchase a number of Equity
Office common shares equal to the number of shares of Cornerstone common stock
that could have been purchased, assuming full vesting, under the corresponding
Cornerstone stock option, multiplied by 0.7009. The exercise price per Equity
Office common share of the substituted option will be equal to the per-share
option exercise price specified in the Cornerstone stock option divided by
0.7009. Each substituted option will otherwise be subject to the same terms and
conditions as the corresponding Cornerstone stock option.

     Holders of Cornerstone stock options also will have the option to surrender
to Equity Office any or all of their options, after the completion of the merger
and before the end of the second business day immediately following the
completion of the merger, in exchange for an amount in cash equal to:

     - the excess, if any, of $18.00 over the exercise price of such Cornerstone
       stock option; multiplied by

     - the number of shares of Cornerstone common stock subject to that
       Cornerstone stock option.

REPRESENTATIONS AND WARRANTIES OF EQUITY OFFICE AND CORNERSTONE

     The merger agreement contains customary representations and warranties by
each of Cornerstone, Cornerstone Partnership, Equity Office and EOP Partnership
relating to, among other things:

     - due organization and good standing;

     - capitalization;

     - authorization to enter into the merger agreement and to consummate the
       merger or the partnership merger, as applicable;

     - enforceability of the merger agreement;

     - no breach of organizational documents or material agreements as a result
       of the merger agreement or the consummation of the merger or the
       partnership merger, as applicable;

     - required governmental and third-party consents;

     - compliance with SEC reporting requirements;

     - no undisclosed liabilities;
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<PAGE>   70

     - no changes since December 31, 1998 that would have a material adverse
       effect;

     - no material legal proceedings;

     - real property;

     - environmental matters;

     - tax matters;

     - finders' fees;

     - compliance with laws;

     - contracts and debt instruments;

     - receipt of opinion of financial advisor;

     - anti-takeover statutes;

     - compliance with the Investment Company Act of 1940; and

     - required stockholder and partner approvals.

     In addition, the merger agreement contains customary representations and
warranties by each of Cornerstone and Cornerstone Partnership relating to, among
other things:

     - related party transactions;

     - employee benefits and labor matters; and

     - payments to employees, officers and directors as a result of the mergers
       or a termination of service after the mergers.

CONDUCT OF BUSINESS OF EQUITY OFFICE AND EOP PARTNERSHIP PENDING THE MERGER

     Pending the merger, each of Equity Office and EOP Partnership has agreed
to, among other things:

     - preserve intact its business organizations and goodwill;

     - use commercially reasonable efforts to keep available the services of its
       officers and employees;

     - confer on a regular basis with Cornerstone to report material operational
       matters that would reasonably be expected to have a material adverse
       effect on the business, properties, assets, financial condition or
       results of operations of Equity Office and its subsidiaries taken as a
       whole;

     - promptly notify Cornerstone of any material emergency or other material
       change in its condition, business, properties, assets, liabilities or
       prospects, or of any material governmental complaints, investigations or
       hearings;

     - promptly deliver to Cornerstone true and correct copies of any report,
       statement or schedule filed with the SEC;

     - maintain its books and records in accordance with GAAP consistently
       applied, and not to change any of its methods, principles or practices of
       accounting in any material manner, except as required by the SEC,
       applicable law or GAAP; and

     - duly and timely file all reports, tax returns and other documents
       required to be filed with federal, state, local and other authorities,
       subject to extensions permitted by law, provided such extensions do not
       adversely affect Equity Office's status as a qualified REIT under the
       Internal Revenue Code.

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<PAGE>   71

     In addition, pending the merger, each of Equity Office and EOP Partnership
has agreed that, without Cornerstone's prior written consent or except as
otherwise expressly contemplated by the merger agreement, neither Equity Office
and EOP Partnership nor any of its subsidiaries will, among other things:

     - make or rescind any express or deemed election relating to taxes unless
       required by law or necessary to preserve Equity Office's status as a REIT
       or the status of any subsidiary of Equity Office as a partnership for
       federal income tax purposes or as a qualified REIT subsidiary as defined
       under the Internal Revenue Code, as the case may be;

     - amend their organizational documents, except in specified instances,
       including the proposed amendments to Equity Office's declaration of trust
       described in this joint proxy statement/prospectus;

     - authorize, declare, set aside or pay any dividend, other than regular
       quarterly dividends, or regular distributions pursuant to the limited
       partnership agreement of EOP Partnership, or redemptions of EOP
       Partnership units under the EOP Partnership agreement where solely Equity
       Office common shares are used, or redemptions of Equity Office common
       shares under its declaration of trust to maintain REIT status or
       repurchases of its common shares pursuant to its publicly announced share
       repurchase program;

     - enter into or agree to effect any merger, acquisition, consolidation,
       reorganization or other business combination with any third party in
       which Equity Office is not the surviving party to the transaction or
       enter into or agree to effect any merger, acquisition, exchange offer or
       other business combination with a third party in which Equity Office is
       the surviving party that would result in the issuance of equity
       securities representing in excess of 20% of the outstanding Equity Office
       common shares on the date any such business combination is entered into
       or agreed to unless, in either case, such business combination is
       approved by Cornerstone, which approval will not be unreasonably withheld
       or delayed, or the business combination agreement provides that the
       required vote of Equity Office shareholders for approval of such business
       combination is no less than the affirmative vote of holders of Equity
       Office common shares representing more than 50% of the sum of the number
       of Equity Office common shares outstanding at the time of such approval
       plus 50,000,000; and

     - authorize, recommend, propose or announce an intention to do any of the
       foregoing prohibited actions, or enter into any contract, agreement,
       commitment or arrangement to do any of the foregoing prohibited actions.

CONDUCT OF BUSINESS OF CORNERSTONE AND CORNERSTONE PARTNERSHIP PENDING THE
MERGER

     Pending the merger, each of Cornerstone and Cornerstone Partnership has
agreed to, among other things:

     - conduct its business, and cause its subsidiaries to conduct their
       respective businesses, only in the ordinary course and in a manner which
       is substantially consistent with past practice;

     - preserve intact its business organizations and goodwill;

     - use commercially reasonable efforts to keep available the services of its
       officers and employees;

     - confer on a regular basis with Equity Office to report material
       operational matters and, except in specified instances, any proposals to
       engage in material transactions;

     - promptly notify Equity Office of any material emergency or other material
       change in its condition, business, properties, assets, liabilities or
       prospects, or of any material governmental complaints, investigations or
       hearings;

     - promptly deliver to Equity Office true and correct copies of any report,
       statement or schedule filed with the SEC;

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     - maintain its books and records in accordance with GAAP consistently
       applied, and not to change any of its methods, principles or practices of
       accounting in any material manner, except as required by the SEC,
       applicable law or GAAP; and

     - duly and timely file all reports, tax returns and other documents
       required to be filed with federal, state, local and other authorities,
       subject to extensions permitted by law, provided that Cornerstone
       notifies Equity Office of any extensions and provided that such
       extensions do not adversely affect Cornerstone's status as a qualified
       REIT under the Internal Revenue Code.

     In addition, pending the merger, each of Cornerstone and Cornerstone
Partnership has agreed that, without Equity Office's prior written consent or
except as otherwise expressly contemplated by the merger agreement, neither
Cornerstone or Cornerstone Partnership nor any of its subsidiaries will, among
other things:

     - make or rescind any express or deemed election relating to taxes unless
       required by law or necessary to preserve Cornerstone's status as a REIT
       or the status of any subsidiary of Cornerstone as a partnership for
       federal income tax purposes or as a qualified REIT subsidiary as defined
       under the Internal Revenue Code, as the case may be;

     - acquire, enter into any option to acquire, or exercise an option or other
       right or election or enter into any commitment for the acquisition of any
       real property or, except as permitted in the budget approved in writing
       by Equity Office, other transaction involving payments to or by
       Cornerstone in excess of $100,000, except in the ordinary course of its
       office property business, including leasing activities permitted by
       Equity Office approved guidelines;

     - encumber assets, commence construction of or enter into any commitment to
       develop or construct real estate projects, except in the ordinary course
       of its office property business;

     - incur or enter into any commitment to incur additional debt, except for
       working capital under its revolving line(s) of credit and commitments for
       debt specified in the schedules to the merger agreement;

     - modify, amend or terminate, or enter into any commitment to modify, amend
       or terminate, any debt in existence on the date of the merger agreement;

     - amend their organizational documents;

     - make any change in the number of shares of capital stock or units of
       limited partnership interest issued and outstanding, except in specified
       instances;

     - grant options or any other rights or commitments relating to its shares
       of capital stock or units of limited partnership interest, or any
       security convertible into its shares of capital stock or units of limited
       partnership interest, or any security the value of which is measured by
       shares of beneficial interest, or any security subordinated to the claim
       of its general creditors;

     - amend or waive any rights under any options or other stock rights, except
       as provided for in the merger agreement;

     - authorize, declare, set aside or pay any dividend or make any other
       distribution or payment, other than regular quarterly dividends, or
       regular distributions pursuant to the limited partnership agreement of
       Cornerstone Partnership, or redemptions of Cornerstone Partnership units
       under the Cornerstone Partnership agreement where solely Cornerstone
       common stock is used, or as necessary to maintain REIT status;

     - directly or indirectly redeem, purchase or acquire any shares of capital
       stock or units of partnership interest or any option, warrant or right to
       acquire, or security convertible into, shares of capital stock or units
       of partnership interest of Cornerstone, other than redemptions of
       Cornerstone Partnership units under the Cornerstone Partnership agreement
       where solely Cornerstone common stock is used, other than the use of
       Cornerstone common stock in connection with equity-based

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       employee benefit plans or as necessary to maintain REIT status and other
       than as set forth in the disclosure letter delivered in connection with
       the merger agreement;

     - sell, lease, mortgage, subject to lien or otherwise dispose of any real
       property, except in specified instances;

     - sell, lease, mortgage, subject to lien or otherwise dispose of any
       personal property or intangible property, except in the ordinary course
       of business and as is not material, individually or in the aggregate;

     - make any loans, advances or capital contributions to, or investments in,
       any other person, except in specified instances;

     - enter into any new, or amend or supplement any existing, contract, lease
       or other agreement with WCP Services, Inc.;

     - pay, discharge or satisfy any claims, liabilities or obligations whether
       absolute, accrued, asserted or unasserted, contingent or otherwise,
       except in specified instances;

     - guarantee the debt of another person, enter into any "keep well" or other
       agreement to maintain any financial statement condition of another person
       or enter into any arrangement having the economic effect of any of the
       foregoing;

     - except in specified instances, enter into any commitment with any
       officer, director or affiliate of Cornerstone or any of its subsidiaries
       or WCP Services, Inc. or any material commitment with any consultant;

     - increase any compensation or enter into or amend any employment agreement
       with any of its officers, directors or employees earning more than
       $100,000 per year, other than as required by any contract or plan or in
       accordance with waivers by employees of benefits under these agreements;

     - adopt any new employee benefit plan or amend any existing plans or
       rights;

     - settle any stockholder derivative or class action claims arising out of
       or in connection with any of the transactions contemplated by the merger
       agreement;

     - change the ownership of any of its subsidiaries or WCP Services, Inc.,
       except in specified instances;

     - accept a promissory note in payment of the exercise price payable under
       any option to purchase shares of Cornerstone common stock;

     - except in specified instances, enter into any oral or written "tax
       protection agreement" that:

       -- has as one of its purposes to permit a person or entity to assert that
          such person or entity could defer federal taxable income that
          otherwise might have been recognized upon a transfer of property to
          the Cornerstone Partnership or any other Cornerstone subsidiary that
          is treated as a partnership for federal income tax purposes, and that
          (a) prohibits or restricts in any manner the disposition of any assets
          of Cornerstone, any Cornerstone subsidiary, including Cornerstone's
          non-controlled subsidiary, (b) requires that Cornerstone, any
          Cornerstone subsidiary or the Cornerstone non-controlled subsidiary
          maintain, or put in place, or replace, indebtedness, whether or not
          secured by one or more of Cornerstone's properties, or (c) requires
          that Cornerstone, any Cornerstone subsidiary or Cornerstone's
          non-controlled subsidiary offer to any person or entity at any time
          the opportunity to guarantee or otherwise assume, directly or
          indirectly (including, without limitation, through a "deficit
          restoration obligation," indemnification agreement or other similar
          arrangement), the risk of loss for federal income tax purposes for
          indebtedness or other liabilities of Cornerstone, any Cornerstone
          subsidiary or Cornerstone's non-controlled subsidiary;

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<PAGE>   74

       -- specifies or relates to a method of taking into account book-tax
          disparities under Section 704(c) of the Internal Revenue Code with
          respect to one or more assets of Cornerstone or a Cornerstone
          subsidiary; or

       -- requires a particular method for allocating one or more liabilities of
          Cornerstone or any Cornerstone subsidiary under Section 752 of the
          Internal Revenue Code.

     - authorize, recommend, propose or announce an intention to do any of the
       foregoing prohibited actions, or enter into any contract, agreement,
       commitment or arrangement to do any of the foregoing prohibited actions.

CONDITIONS TO THE MERGER AND THE PARTNERSHIP MERGER

CONDITIONS TO EACH PARTY'S OBLIGATIONS TO EFFECT THE MERGER AND THE PARTNERSHIP
MERGER

     The obligations of Cornerstone, Cornerstone Partnership, Equity Office and
EOP Partnership to complete the merger and the partnership merger are subject to
the satisfaction or, where permissible, waiver of the following conditions:

     - the approval of the merger and the partnership merger, as applicable, and
       the merger agreement by the affirmative vote of the holders of at least:

       -- a majority of the outstanding shares of Cornerstone common stock;

       -- a majority of the outstanding Equity Office common shares;

       -- two-thirds of the outstanding shares of Cornerstone preferred stock;

       -- a majority of all limited partner interests in Cornerstone Partnership
          entitled to be cast, in accordance with the Cornerstone Partnership
          agreement; and

       -- a majority of all partner interests in EOP Partnership, including
          partner interests held by Equity Office.

     - the applicable waiting period under the Hart-Scott-Rodino Antitrust
       Improvements Act of 1976 will have expired or been terminated;

     - the New York Stock Exchange will have approved for listing the Equity
       Office common shares to be issued in the merger and the Equity Office
       common shares reserved for issuance upon redemption of EOP Partnership
       units issued in the partnership merger, subject to official notice of
       issuance;

     - the registration statement on Form S-4 of which this proxy
       statement/prospectus forms a part will have become effective and will not
       be the subject of any stop order or proceedings by the SEC seeking a stop
       order;

     - no temporary restraining order, preliminary or permanent injunction or
       other order issued by any court of competent jurisdiction or other legal
       restraint or prohibition preventing the completion of the mergers or any
       of the other transactions contemplated by the merger agreement will be in
       effect; and

     - Equity Office and EOP Partnership will have received all state securities
       or "blue sky" permits and other authorizations necessary to issue the
       Equity Office common shares issuable in the merger and the EOP
       Partnership units issuable in the partnership merger.

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CONDITIONS TO THE OBLIGATIONS OF EQUITY OFFICE AND EOP PARTNERSHIP TO EFFECT THE
MERGER AND THE PARTNERSHIP MERGER

     The obligations of Equity Office and EOP Partnership to complete the
mergers are subject to the satisfaction or, where permissible, waiver of the
following conditions:

     - each of the representations and warranties of Cornerstone and Cornerstone
       Partnership contained in the merger agreement, disregarding all
       qualifications and exceptions relating to "materiality" or "material
       adverse effect," will be true and correct as of the date of the merger
       agreement and as of the closing of the merger:

       -- except to the extent that these representations and warranties are
          expressly limited by their terms to another date, in which case these
          representations and warranties will be true and correct as of that
          other date; and

       -- except where the failure of these representations and warranties to be
          true and correct would not, individually or in the aggregate,
          reasonably be expected to have a material adverse effect on the
          business, properties, assets, financial condition or results of
          operations of Cornerstone and its subsidiaries taken as a whole.

     - Cornerstone and Cornerstone Partnership will have performed in all
       material respects all obligations required to be performed by them under
       the merger agreement at or before the completion of the merger;

     - since February 11, 2000, there will have been no material adverse change
       in the business, financial condition or results of operations of
       Cornerstone and its subsidiaries taken as a whole;

     - Equity Office will have received a certificate of an officer of
       Cornerstone certifying to each of the above;

     - Equity Office will have received an opinion, dated as of the closing date
       of the merger:

       -- from counsel to Cornerstone relating to the REIT status of Cornerstone
          and the partnership status of Cornerstone Partnership;

       -- from counsel to Equity Office relating to the REIT status of Equity
          Office; and

       -- from counsel to Equity Office relating to the federal income tax
          treatment of the merger.

     - Equity Office and EOP Partnership will have received a "comfort" letter
       with respect to this document from independent public accountants for
       Cornerstone and Cornerstone Partnership; and

     - each of William Wilson III and John S. Moody will have entered into
       noncompetition and confidentiality agreements.

CONDITIONS TO THE OBLIGATIONS OF CORNERSTONE AND CORNERSTONE PARTNERSHIP TO
EFFECT THE MERGER AND THE PARTNERSHIP MERGER

     The obligations of Cornerstone and Cornerstone Partnership to complete the
merger and the partnership merger are subject to the satisfaction or, where
permissible, waiver of the following conditions:

     - each of the representations and warranties of Equity Office and EOP
       Partnership contained in the merger agreement, disregarding all
       qualifications and exceptions relating to "materiality" or "material
       adverse effect," will be true and correct as of the date of the merger
       agreement and as of the closing of the merger:

       -- except to the extent that these representations and warranties are
          expressly limited by their terms to another date, in which case these
          representations and warranties will be true and correct as of that
          other date; and

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       -- except where the failure of these representations and warranties to be
          true and correct would not reasonably be expected to have a material
          adverse effect on the business, properties, assets, financial
          condition or results of operations of Equity Office and its
          subsidiaries taken as a whole.

     - Equity Office and EOP Partnership will have performed in all material
       respects all obligations required to be performed by them under the
       merger agreement at or before the completion of the merger;

     - since the date of the merger agreement, there will have been no material
       adverse change in the business, financial condition or results of
       operations of Equity Office and its subsidiaries taken as a whole;

     - Cornerstone will have received a certificate of an officer of Equity
       Office certifying to each of the foregoing;

     - Cornerstone will have received an opinion, dated as of the closing date
       of the merger:

       -- from counsel to Equity Office relating to the REIT status of Equity
          Office and BeaMetFed, Inc., a Maryland corporation in which EOP
          Partnership currently owns 51.6% of the outstanding stock, and the
          partnership status of EOP Partnership; and

       -- from counsel to Cornerstone relating to the federal income tax
          treatment of the merger.

     - The Equity Office shareholders will have approved the proposed Equity
       Office charter amendment relating to domestically controlled REIT status
       at the Equity Office special meeting;

     - Cornerstone and Cornerstone Partnership will have received a "comfort"
       letter with respect to this document from independent public accountants
       for Equity Office and EOP Partnership; and

     - Equity Office will have delivered to PGGM a certificate to the effect
       that, to Equity Office's knowledge, Equity Office is a "domestically
       controlled REIT" as defined in the Internal Revenue Code.

NO SOLICITATION BY CORNERSTONE

     Cornerstone has agreed, for itself and in its capacity as the sole general
partner of Cornerstone Partnership, that neither it nor any Cornerstone
subsidiary will, nor will any of them permit any officer, director, employee,
affiliate, agent, investment banker, financial advisor, attorney, accountant,
broker, finder, consultant or other agent or representative to:

     - invite, initiate, solicit or encourage, directly or indirectly, any
       inquiries, proposals, discussions or negotiations or the making or
       implementation of any proposal or offer, including any proposal or offer
       to its stockholders, with respect to an "alternative acquisition
       proposal," as defined below;

     - engage in any discussions or negotiations with or provide any
       confidential or non-public information or data to, any person relating
       to, or that may reasonably be expected to lead to, an alternative
       acquisition proposal;

     - enter into any letter of intent, agreement in principle or agreement
       relating to an alternative acquisition proposal;

     - propose publicly to agree to do any of the foregoing; or

     - otherwise facilitate any effort or attempt to make or implement an
       alternative acquisition proposal.

     Cornerstone has agreed, for itself and in its capacity as the sole general
partner of Cornerstone Partnership, that it will:

     - immediately cease and cause to be terminated any existing activities,
       discussions or negotiations with any parties conducted previously with
       respect to any alternative acquisition proposal; and

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     - notify Equity Office promptly if Cornerstone or any of its subsidiaries
       receives any such inquiries or proposals, or any requests for such
       information, or if any such negotiations or discussions are sought to be
       initiated or continued with it, and include in such notice the identity
       of the person making such inquiry, proposal or request, the material
       terms of such inquiry, proposal or request and, if in writing, will
       promptly deliver to Equity Office a copy of such inquiry, proposal or
       request along with all other related documentation and correspondence.

     Notwithstanding the foregoing, Cornerstone, including in its capacity as
the sole general partner of Cornerstone Partnership, may furnish information to,
or enter into discussions or negotiations with, any person that makes an
unsolicited bona fide written proposal relating to an alternative acquisition
proposal, if, and only to the extent that:

     - a majority of the board of directors of Cornerstone determines in good
       faith, after consultation with its outside counsel, that such action is
       required for the board of directors of Cornerstone to comply with its
       fiduciary duties to stockholders imposed by applicable law;

     - before furnishing such information to, or entering into discussions or
       negotiations with, such person, Cornerstone provides written notice to
       Equity Office to the effect that it is furnishing information to, or
       entering into discussions with, such person; and

     - Cornerstone enters into a confidentiality agreement with that person on
       terms in the aggregate not more favorable to that person than the terms
       of the confidentiality agreement entered into with Equity Office.

     For purposes of the merger agreement, an "alternative acquisition proposal"
means a merger, acquisition, tender offer, exchange offer, transaction resulting
in the issuance of securities representing 10% or more of the outstanding equity
securities of Cornerstone or Cornerstone Partnership, consolidation, business
combination, recapitalization, liquidation, dissolution, share exchange,
business combination, sale, lease, exchange, mortgage, pledge, transfer or other
disposition of 10% or more of the assets or equity securities, including
partnership interests and units, of Cornerstone or Cornerstone Partnership,
other than the mergers.

     Notwithstanding the foregoing, in the event that an alternative acquisition
proposal constitutes a superior alternative acquisition proposal, the board of
directors of Cornerstone may withdraw, modify, amend or qualify its
recommendation of this merger agreement and the merger and recommend that
superior alternative acquisition proposal to its stockholders if:

     - Cornerstone complies fully with the foregoing provisions and provides
       Equity Office with at least three business days' prior written notice of
       its intent to withdraw, modify, amend or qualify its recommendation of
       the merger agreement and the merger;

     - during such three business days Equity Office makes a counter proposal to
       such superior alternative acquisition proposal, Cornerstone's board of
       directors in good faith, taking into account the advice of its outside
       financial advisors of nationally recognized reputation, determines that
       the Equity Office counter proposal is:

       -- not at least as favorable to Cornerstone's stockholders as the
          superior alternative acquisition proposal, from a financial point of
          view; and

       -- not at least as favorable generally to Cornerstone's stockholders,
          taking into account all financial and strategic considerations and
          other relevant factors, including relevant legal, financial,
          regulatory and other aspects of that proposal, and the conditions,
          prospects and time required for completion of that proposal; and

     - Cornerstone terminates the merger agreement in accordance with its terms
       and pays to EOP Partnership the termination fee.

     For purposes of the merger agreement, a "superior alternative acquisition
proposal" means a bona fide written proposal made by a third party to acquire,
directly or indirectly, Cornerstone and/or Cornerstone
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Partnership pursuant to a tender or exchange offer, merger, share exchange,
consolidation or sale of all or substantially all of the assets of Cornerstone,
Cornerstone Partnership, and their subsidiaries or otherwise:

     - on terms which a majority of the board of directors of Cornerstone
       determines in good faith, taking into account the advice of Cornerstone's
       financial advisors of nationally recognized reputation, are superior,
       from a financial point of view, to Cornerstone's stockholders to those
       provided for in the merger;

     - on terms which a majority of the board of directors of Cornerstone
       determines in good faith to be more favorable generally to Cornerstone's
       stockholders, taking into account all financial and strategic
       considerations and other relevant factors, including relevant legal,
       financial, regulatory and other aspects of such proposals, and the
       conditions, prospects and time required for completion of such proposal;

     - for which financing, to the extent required, is then fully committed and
       capable of being obtained; and

     - which the board of directors of Cornerstone determines in good faith is
       reasonably capable of being consummated.

TERMINATION OF THE MERGER AGREEMENT

  RIGHT TO TERMINATE

     The merger agreement may be terminated at any time before the completion of
the partnership merger, whether before or after approval of the merger agreement
and the mergers by the Equity Office shareholders, EOP Partnership limited
partners, Cornerstone stockholders or Cornerstone limited partners, as follows:

     - by mutual written consent duly authorized by the Equity Office board of
       trustees and the Cornerstone board of directors;

     - by either Equity Office or Cornerstone if:

       -- any judgment, injunction, order, decree or action by any governmental
          entity preventing the consummation of either the merger or the
          partnership merger becomes final and non-appealable;

       -- the merger and the partnership merger have not been completed before
          December 31, 2000; however, neither Equity Office nor Cornerstone may
          terminate the merger agreement if its breach is the reason that the
          mergers have not been completed;

       -- the holders of at least a majority of the outstanding shares of
          Cornerstone common stock and two-thirds of the outstanding shares of
          Cornerstone preferred stock fail to approve the merger and the merger
          agreement at the Cornerstone special meeting, or if holders of a
          majority of the limited partner interests in Cornerstone Partnership
          fail to approve the merger agreement and the partnership merger, but
          Cornerstone may not terminate for either of these reasons if it is in
          breach in any material respect of its obligations contained in the
          merger agreement relating to obtaining the required shareholder and
          partner votes; or

       -- the holders of Equity Office common shares fail to approve the merger
          and the merger agreement at the Equity Office special meeting, or
          holders of a majority of the partner interests in EOP Partnership fail
          to approve the merger agreement and the partnership merger, but Equity
          Office may not terminate for either of these reasons if it is in
          breach in any material respect of its obligations contained in the
          merger agreement relating to obtaining the required shareholder and
          partner votes.

     - by Equity Office:

       -- upon a breach of or failure to perform any representation, warranty,
          covenant, obligation or agreement on the part of Cornerstone or
          Cornerstone Partnership contained in the merger

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agreement, or if any representation or warranty of Cornerstone or Cornerstone
Partnership becomes untrue, in either case such that the conditions to the
consummation of the merger contained in the merger agreement would be incapable
        of being satisfied by December 31, 2000, or as otherwise extended by the
        parties; or

       -- if (a) the Cornerstone board of directors has failed to recommend or
          has withdrawn, modified, amended or qualified, or proposed publicly
          not to recommend or to withdraw, modify, amend or qualify, in any
          manner adverse to Equity Office its approval or recommendation of
          either of the mergers or the merger agreement or approved or
          recommended any superior alternative acquisition proposal, (b)
          following the announcement or receipt of an alternative acquisition
          proposal, Cornerstone has failed to call the Cornerstone special
          meeting or failed to prepare and mail to its stockholders this joint
          proxy statement/prospectus or (c) the Cornerstone board of directors
          or any committee of the Cornerstone board of directors has resolved to
          do any of the foregoing.

     - by Cornerstone:

       -- upon a breach of any representation, warranty, covenant obligation or
          agreement on the part of Equity Office or EOP Partnership contained in
          the merger agreement, or if any representation or warranty of Equity
          Office or EOP Partnership becomes untrue, in either case such that the
          conditions to the consummation of the merger contained in the merger
          agreement would be incapable of being satisfied by December 31, 2000
          or as otherwise extended by the parties; or

       -- if the Cornerstone board of directors has withdrawn, modified, amended
          or qualified in any manner adverse to Equity Office its approval or
          recommendation of either of the merger or the merger agreement in
          connection with, or approved or recommended, any superior alternative
          acquisition proposal, or in order to enter into a binding written
          agreement with respect to a superior alternative acquisition proposal,
          so long as, in each case, Cornerstone complied with the terms of the
          no solicitation provisions contained in the merger agreement and,
          before terminating the merger agreement, has paid to EOP Partnership
          the termination fee.

EFFECT OF TERMINATION

     Except for provisions in the merger agreement regarding confidentiality of
nonpublic information, payment of fees and expenses, the effect of termination
and specified miscellaneous provisions, if the merger agreement is terminated as
described above, the merger agreement will become void and have no effect. In
addition, if the merger agreement is so terminated, there will be no liability
on the part of Equity Office, EOP Partnership, Cornerstone or Cornerstone
Partnership, except to the extent that such termination results from a material
breach by any party of any of its representations, warranties, covenants or
agreements contained in the merger agreement. The confidentiality agreement,
dated January 13, 2000, between Cornerstone and Equity Office will continue in
effect notwithstanding any termination of the merger agreement.

EXPENSES; TERMINATION FEE

     Except as described below, each party to the merger agreement will bear its
own fees and expenses in connection with the transactions contemplated by the
merger agreement.

     Cornerstone and Cornerstone Partnership will pay to EOP Partnership a
termination fee if the merger agreement is terminated:

     - by Cornerstone, (a) if the Cornerstone board of directors has withdrawn,
       modified, amended or qualified in any manner adverse to Equity Office its
       approval or recommendation of either of the merger or the merger
       agreement in connection with, or approved or recommended, any superior
       alternative acquisition proposal, or (b) in order to enter into a binding
       written agreement with respect to a superior alternative acquisition
       proposal, so long as, in each case, Cornerstone complied with the terms
       of the nonsolicitation provisions contained in the merger agreement;
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     - by Equity Office if the Cornerstone board of directors has failed to
       recommend or has withdrawn, modified, amended or qualified, or proposed
       publicly not to recommend or to withdraw, modify, amend or qualify, in
       any manner adverse to Equity Office its approval or recommendation of
       either of the mergers or the merger agreement or approved or recommended
       any superior alternative acquisition proposal, or has resolved to do any
       of the foregoing; or

     - by either party under the circumstances listed below, but only if (a)
       Cornerstone or Cornerstone Partnership has received a proposal for an
       alternative acquisition transaction before such termination and (b)
       before or within 12 months after such termination, Cornerstone or
       Cornerstone Partnership enters into an agreement regarding any
       alternative acquisition transaction that is later completed:

       -- Cornerstone breaches any of its representations or warranties in the
          merger agreement, and such breach has a material adverse effect on
          Cornerstone and its subsidiaries taken as a whole and cannot be
          rectified by December 31, 2000;

       -- Cornerstone fails to perform in all material respects all of its
          covenants, obligations or agreements in the merger agreement and such
          failure cannot be rectified by December 31, 2000;

       -- following Cornerstone's receipt or announcement of an alternative
          acquisition proposal, Cornerstone fails to call the Cornerstone
          special meeting or fails to prepare and mail to its stockholders this
          joint proxy statement/prospectus;

       -- any judgment, injunction, order, decree or action by any governmental
          entity of competent authority, which primarily results from any
          action, inaction of Cornerstone, its subsidiaries or its
          non-controlled subsidiary and prevents the consummation of the merger
          or the partnership merger, becomes final and non-appealable;

       -- the merger and the partnership merger are not completed before
          December 31, 2000 and the terminating party has not materially
          breached its obligations under the merger agreement in a way that
          prevents the merger or the partnership merger from being completed
          before December 31, 2000; or

       -- the holders of at least a majority of the outstanding shares of
          Cornerstone common stock and the holders of at least two-thirds of the
          outstanding shares of Cornerstone preferred stock fail to approve the
          merger and the merger agreement at the Cornerstone special meeting.

     The termination fee that EOP Partnership may be entitled to receive will be
an amount equal to the lesser of (1) $100 million less termination expenses, as
described below, paid or payable under the merger agreement and (2) the maximum
amount that can be paid to EOP Partnership without causing Equity Office to fail
to meet the REIT income requirements under the Internal Revenue Code.
Cornerstone's and Cornerstone Partnership's obligation to pay any unpaid portion
of the termination fee will terminate on February 11, 2003.

     Cornerstone and Cornerstone Partnership will pay to EOP Partnership
termination expenses if the merger agreement is terminated:

     - by Equity Office, upon a breach of or failure to perform any
       representation, warranty, covenant, obligation or agreement on the part
       of Cornerstone or Cornerstone Partnership contained in the merger
       agreement, or if any representation or warranty of Cornerstone or
       Cornerstone Partnership becomes untrue, in either case such that the
       conditions to the consummation of the merger contained in the merger
       agreement would be incapable of being satisfied by December 31, 2000, or
       as otherwise extended by the parties, so long as Cornerstone was not
       entitled to terminate the merger agreement because of breach of any
       representation, warranty, covenant obligation or agreement on the part of
       Equity Office or EOP Partnership; or

     - by either Equity Office or Cornerstone, if the holders of a majority of
       the outstanding shares of Cornerstone common stock and Cornerstone
       preferred stock fail to approve the mergers and the

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       merger agreement at the Cornerstone special meeting, or if holders of a
       majority of the limited partner interests in Cornerstone Partnership fail
       to approve the merger agreement and the partnership merger, so long as
       Cornerstone was not entitled to terminate the merger agreement because of
       breach of any representation, warranty, covenant obligation or agreement
       on the part of Equity Office or EOP Partnership.

     Equity Office and EOP Partnership will pay to Cornerstone Partnership
termination expenses if the merger agreement is terminated:

     - by Cornerstone, upon a breach of any representation, warranty, covenant
       obligation or agreement on the part of Equity Office or EOP Partnership
       contained in the merger agreement, or if any representation or warranty
       of Equity Office or EOP Partnership becomes untrue, in either case such
       that the conditions to the consummation of the merger contained in the
       merger agreement would be incapable of being satisfied by December 31,
       2000 or as otherwise extended by the parties, so long as Equity Office
       was not entitled to terminate the merger agreement because of breach of
       any representation, warranty, covenant obligation or agreement on the
       part of Cornerstone or Cornerstone Partnership; or

     - by either Cornerstone or Equity Office, if the holders of a majority of
       the outstanding Equity Office common shares fail to approve the mergers
       and the merger agreement at the Equity Office special meeting, or if
       holders of a majority of the partner interests in EOP Partnership fail to
       approve the merger agreement and the partnership merger, but neither
       Equity Office nor Cornerstone may terminate for either of these reasons
       if it is in breach in any material respect of its obligations contained
       in the merger agreement relating to obtaining the required shareholder
       and partner votes, so long as Equity Office was not entitled to terminate
       the merger agreement because of breach of any representation, warranty,
       covenant obligation or agreement on the part of Cornerstone or
       Cornerstone Partnership.

     The termination expenses that EOP Partnership or Cornerstone Partnership
may be entitled to receive will be an amount equal to the lesser of (a)
$7,500,000 or (b) such party's out-of-pocket expenses incurred in connection
with the merger agreement and the transactions contemplated by the merger
agreement, including all attorneys', accountants' and investment bankers' fees
and expenses. If the termination expenses payable to that party exceed the
maximum amount that can be paid to that party without causing that party to fail
to meet the REIT income requirements under the Internal Revenue Code, then the
amount initially payable to that party will be that maximum amount. The paying
party's obligation to pay any unpaid portion of the termination fee will
terminate on February 11, 2003.

     In addition, in the event that Equity Office prevails in a suit for a
breach by Cornerstone and Cornerstone Partnership of their obligation to pay the
termination fee or termination expenses under the merger agreement, Equity
Office will be entitled to its costs and expenses in connection with the suit,
with interest.

WAIVER AND AMENDMENT OF THE MERGER AGREEMENT

     The merger agreement may be amended by the parties in writing by action of
the Equity Office board of trustees and the Cornerstone board of directors at
any time before the filing of the Maryland articles of merger.

     At any time before the completion of the merger, the parties may, in
writing:

     - extend the time for the performance of any of the obligations or other
       acts of the other party;

     - waive any inaccuracies in the representations and warranties of the other
       party contained in the merger agreement or in any document delivered
       pursuant to the merger agreement; or

     - waive compliance with any of the agreements or conditions of the other
       party contained in the merger agreement, except as specified.

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<PAGE>   82

INDEMNIFICATION; DIRECTORS' AND OFFICERS' INSURANCE

     In the merger agreement, Equity Office and EOP Partnership have agreed to
provide exculpation and indemnification for each person who has been at any time
on or before February 11, 2000, or who becomes before the completion of the
merger, an officer or director of Cornerstone or any Cornerstone subsidiary.
This exculpation and indemnification will be the same as provided to these
persons by Cornerstone and its subsidiaries immediately before the completion of
the merger in each entity's respective charter, bylaws, partnership, operating
or similar agreement, as applicable, as in effect on February 11, 2000. This
exculpation and indemnification covers actions only on or before the completion
of the merger, including all transactions contemplated by the merger agreement.

     In addition, Equity Office and EOP Partnership also agreed to indemnify and
hold harmless, to the full extent permitted by applicable law, each of the
persons described above against any losses, claims, liabilities, expenses,
judgments, fines and amounts paid in settlement in connection with any
threatened or actual claim, action, suit, proceeding or investigation, whether
civil, criminal or administrative, including any action by or on behalf of any
or all security holders of Cornerstone or Equity Office, or any of their
subsidiaries, or by or in the right of Cornerstone or Equity Office, or any of
their subsidiaries, in which any of these persons is, or is threatened to be,
made a party based in whole or in part on, or arising in whole or in part out
of, or pertaining to:

     - the fact that he or she is or was an officer, employee or director of
       Cornerstone or any of its subsidiaries or any action or omission by that
       person in his capacity as a director; or

     - the merger agreement or the transactions contemplated by the merger
       agreement, whether in any case asserted or arising before or after the
       completion of the merger.

     After the completion of the merger, Equity Office and EOP Partnership will
be obligated to promptly pay and advance reasonable expenses and costs incurred
by each of these persons as they become due and payable in advance of the final
disposition of the claim, action, suit, proceeding or investigation to the
fullest extent and in the manner permitted by law. Equity Office is also
obligated to purchase, at or before the completion of the merger, directors'
liability insurance policy coverage for Cornerstone's directors and executive
officers for a period of six years which will provide the directors and officers
with coverage on substantially similar terms as currently provided by
Cornerstone to these directors and officers.

ASSUMPTION OF CORNERSTONE'S OBLIGATIONS UNDER REGISTRATION RIGHTS AGREEMENTS

     Under the merger agreement, Equity Office has agreed to assume
Cornerstone's obligations under existing registration rights agreements between
Cornerstone and certain holders of Cornerstone Partnership units.

     In addition, under the merger agreement, Equity Office has agreed to assume
Cornerstone's obligations under the amended and restated registration rights and
voting agreement, dated December 16, 1998, among Cornerstone, PGGM and Dutch
Institutional Holding Company, Inc., as amended by the voting agreement entered
into concurrently with the merger agreement among Equity Office, EOP
Partnership, WCP Services, Inc. and PGGM. Under this registration rights
agreement, PGGM will have the following rights relating to the registration of
Equity Office common shares:

     - Demand Registration: subject to specified limitations, PGGM will have the
       right to demand, on no more than eight occasions, that Equity Office
       register all or a portion of the Equity Office common shares issued to
       PGGM under the merger agreement, subject to a requirement that the market
       value of the shares requested to be registered as of the date of any
       demand must be equal to or greater than (a) $75 million or (b) the total
       aggregate amount of shares held by PGGM, if less than $75 million. Equity
       Office must use its commercially reasonable efforts to effect any such
       demand registration and will have the ability to defer the filing of a
       registration statement relating to a demand in specified circumstances;

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<PAGE>   83

     - Shelf Registration: within 20 days after the request of PGGM following
       the effective time of the merger, Equity Office will use its commercially
       reasonable efforts to register all of the Equity Office common shares
       issued to PGGM in connection with the merger agreement on a registration
       statement which provides for the offering of such securities on a delayed
       or continuous basis. Before selling Equity Office common shares under
       such registration statement, PGGM must give written notice to Equity
       Office of its intent to sell Equity Office common shares. Equity Office
       will have the right in specified circumstances to defer such intended
       sales;

     - "Piggyback" Registration: in the event that Equity Office exercises its
       rights to defer a requested registration, or suspend the effectiveness of
       the shelf registration or sales of Equity Office common shares by PGGM
       due to a contemplated public offering of Equity Office common shares,
       PGGM will have the right to include its shares in Equity Office's
       registration statement relating to such public offering. These
       "piggyback" registration rights will be subject to specified customary
       limitations as well as limitations contained in the other agreements
       between Equity Office and its stockholders.

     All registrations of PGGM shares described above will be at the expense of
Equity Office, except that PGGM will bear all underwriting discounts and
commissions and fees and expenses of its own counsel.

     In addition under the amended and restated registration rights and voting
agreement, PGGM has agreed to a "standstill" provision which provides that PGGM
will not take any of the following actions until 90 days after the date on which
no Equity Office trustee nominated by PGGM in accordance with the PGGM voting
agreement described below on page 72 remains a trustee of Equity Office:

     - directly or indirectly purchase or otherwise acquire, or propose or offer
       to purchase or otherwise acquire, any of Equity Office's equity
       securities if, immediately after such purchase or acquisition, PGGM and
       its affiliates would own 12% or more of the Equity Office common shares.
       "Equity security" means any (a) Equity Office common shares, (b)
       securities of Equity Office that are convertible into or exchangeable for
       Equity Office common shares, and (c) options, rights, warrants and
       similar securities issued by Equity Office to acquire Equity Office
       common shares;

     - directly or indirectly propose to Equity Office or any person or entity a
       "business combination," which is defined to mean any one of the following
       transactions: (a) any merger or consolidation of Equity Office or any
       Equity Office subsidiary with any person or entity, other than Equity
       Office; (b) any sale, lease exchange, mortgage, pledge, transfer or other
       disposition by Equity Office, in one transaction or a series of
       transactions to or with any person or entity, of all or a substantial
       portion of the assets of Equity Office and its subsidiaries taken as a
       whole; (c) the adoption of any plan or proposal for the liquidation or
       dissolution of Equity Office proposed by or on behalf of PGGM or any
       affiliate of PGGM; or (d) any reclassification of securities (including
       any reverse stock split), recapitalization of Equity Office, or any
       merger or consolidation of Equity Office with any of its subsidiaries, or
       any other transaction to which Equity Office is a party which has the
       effect, directly or indirectly, of increasing the proportionate share of
       PGGM or any affiliate of PGGM, whether or not with or into or otherwise
       involving PGGM or any affiliate of PGGM;

     - make, or in any way participate in, any "solicitation" of "proxies" to
       vote, as those terms are used in the rules promulgated by the Securities
       Exchange Commission under Section 14(a) of the Securities Exchange Act of
       1934, or seek to advise, encourage or influence any person or entity with
       respect to the voting of any shares of capital stock of Equity Office,
       initiate, propose or otherwise solicit stockholders of Equity Office for
       the approval of one or more stockholder proposals or induce or attempt to
       induce any other person or entity to initiate any stockholder proposal;
       or

     - deposit any equity securities of Equity Office into a voting trust or
       subject any equity securities to any arrangement or agreement with
       respect to the voting of such securities, or form, join or in any way
       participate in a "group," within the meaning of Section 13(d)(3) of the
       Securities and Exchange Act of 1934, with respect to any equity
       securities, except as contemplated by the registration rights agreement.

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<PAGE>   84

     The standstill provision may be waived by Equity Office only upon the
approval of a majority of its board, excluding any trustee nominated by PGGM,
and is not applicable to actions approved by the majority of the Equity Office
board, excluding any trustee nominated by PGGM in circumstances in which any
trustee nominated by PGGM is an "interested director" under Section 2-419 of the
Maryland General Corporation Law.

VOTING AGREEMENTS

CORNERSTONE VOTING AGREEMENTS

     Each of the directors and executive officers of Cornerstone, including
their affiliates, and PGGM have entered into voting agreements with Equity
Office and EOP Partnership agreeing to vote all shares of Cornerstone common
stock, and, if applicable, all Cornerstone Partnership units, owned of record by
each of them, or that they otherwise have the power to vote:

     - for adoption and approval of the merger agreement and the partnership
       merger, as applicable; and

     - against adoption or approval of any action or agreement made or taken in
       opposition to or in competition with the merger or the partnership
       merger, as applicable.

     Each such stockholder also is restricted from soliciting alternative
acquisition proposals with respect to Cornerstone or engaging in any discussions
or negotiations with, or providing any confidential or non-public information
to, any person relating to an alternative acquisition proposal, or otherwise
facilitate any effort or attempt to make or implement an alternative acquisition
proposal.

     As of the record date for the Cornerstone special meeting, these persons
and entities beneficially owned, excluding stock options and Cornerstone
Partnership units held by them,                shares of Cornerstone common
stock, representing      % of the outstanding shares of Cornerstone common stock
entitled to be voted at the Cornerstone special meeting.

     PGGM Voting Agreement.  In addition to the general terms and conditions
applicable to each of the voting agreements, the voting agreement between Equity
Office, EOP Partnership, WCP Services, Inc. and PGGM also provides, among other
things, that:

     - Equity Office will assume Cornerstone's obligations under the amended and
       restated registration rights and voting agreement, dated December 16,
       1998, among Cornerstone, PGGM and Dutch Institutional Holding Company,
       Inc., as amended by the PGGM voting agreement. The terms of the amended
       and restated registration rights and voting agreement, as they will apply
       after the merger, are described above in the section entitled "Assumption
       of Cornerstone's Obligations under Registration Rights Agreements" on
       page 70.

     - Jan H.W.R. van der Vlist, a current Cornerstone board member, will become
       a trustee of the combined entity, with his term expiring in 2003, and
       Equity Office is obligated to nominate Mr. van der Vlist for reelection
       to the board of trustees in 2003 and 2006 if PGGM and its affiliates
       continue to own at least 21,000,000 or more of the issued and outstanding
       Equity Office common shares, as adjusted for stock splits or similar
       actions, at all times up to the time the trustees are being elected in
       those years. If Mr. van der Vlist fails to stand for re-election in 2003
       or 2006 for any reason, or in the event of his death or earlier
       resignation, and if PGGM continues to own at least 21,000,000 or more of
       the issued and outstanding Equity Office shares as described above,
       Equity Office is required take all action necessary to nominate a
       replacement designated by PGGM for election or re-election as a trustee
       for an additional three-year term, subject to Equity Office's approval if
       the replacement is not an officer, director or employee of PGGM.

EQUITY OFFICE VOTING AGREEMENTS

     Each of the trustees and executive officers of Equity Office, including
their affiliates, have entered into voting agreements with Cornerstone and
Cornerstone Partnership agreeing to vote all Equity Office

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<PAGE>   85

common shares and, if applicable, all EOP Partnership units owned of record by
each of them, or that they otherwise have the power to vote:

     - in favor of adoption and approval of the merger agreement and the merger
       and the partnership merger, as applicable; and

     - against approval or adoption of any action or agreement made or taken in
       opposition to or in competition with the merger or the partnership
       merger, as applicable.

     As of the record date for the Equity Office special meeting, these persons
and entities beneficially owned, excluding share options and EOP Partnership
units held by them,                Equity Office common shares, representing
     % of the outstanding Equity Office common shares entitled to be voted at
the Equity Office special meeting.

PREFERRED STOCK OPTION AGREEMENT

     The holders of all of the outstanding shares of Cornerstone preferred stock
have entered into a stock option agreement with Equity Office and Cornerstone.
Under the agreement, these holders have granted to Cornerstone and Equity
Office, respectively, an option to acquire all of their shares of Cornerstone
preferred stock at any time before the completion of the merger at a purchase
price of $18.00 per share, plus accrued and unpaid dividends, in cash, subject
to adjustment as provided in the agreement. If the merger agreement is amended
to increase the purchase price per share in the merger, either before or after
Cornerstone or Equity Office exercises its option under the agreement to
purchase all of their shares of Cornerstone preferred stock, the purchase price
of the Cornerstone preferred stock will be adjusted to equal the purchase price
contained in the amended merger agreement.

ACQUISITION OF WCP SERVICES

     William Wilson III and John S. Moody are the owners of common shares of
Cornerstone's third-party service subsidiary, WCP Services, Inc., representing
99% of the voting power and 5% of the economic interests in WCP Services.
Messrs. Wilson and Moody have entered into a stock purchase agreement relating
to such common shares of WCP Services. This agreement provides for the sale of
the voting capital stock of WCP Services owned by Messrs. Wilson and Moody to
EOP Office Properties Management Corp., a Delaware corporation in which Equity
Office owns a 95% non-voting equity interest. Each of Messrs. Wilson and Moody
will receive $200,000, including the assumption of indebtedness, in exchange for
his voting capital stock of WCP Services. WCP Services provides third party
management services, tenant services and development services.

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                        COMPARISON OF SHAREHOLDER RIGHTS

     The following is a summary of the material differences between the rights
of holders of Equity Office common shares and holders of Cornerstone common
stock. As a Maryland real estate investment trust, Equity Office is subject to
Maryland law governing real estate investment trusts organized in Maryland. The
Maryland real estate investment trust law covers some of the same matters
covered by the Maryland General Corporation Law, including liabilities of
trustees and officers, amendment of the declaration of trust and mergers of a
Maryland real estate investment trust with other entities. There are many
corporate governance matters that are addressed in the Maryland General
Corporation Law, however, that are not dealt with in the Maryland real estate
investment trust law. It is the general practice of Maryland real estate
investment trusts to address some of these matters through provisions in the
declaration of trust or bylaws. As a Nevada corporation, Cornerstone is subject
to applicable provisions of the Nevada General Corporation Law applicable to
corporations organized in Nevada.

     The following discussion is not intended to be complete and is qualified by
reference to the declaration of trust and bylaws of Equity Office, the articles
of incorporation and bylaws of Cornerstone and applicable Maryland and Nevada
law. The declaration of trust and bylaws of Equity Office and the articles of
incorporation and bylaws of Cornerstone are incorporated by reference in this
joint proxy statement/prospectus, and will be sent to holders of Equity Office
common shares and Cornerstone common stock upon request. See "Where You Can Find
More Information."

AUTHORIZED SHARES

     The authorized shares of beneficial interest of Equity Office consist of
750,000,000 common shares, par value $.01 per share, and 100,000,000 preferred
shares, of which 8,000,000 shares are designated as "8.98% series A cumulative
redeemable preferred shares," 7,000,000 shares are designated as "5.25% series B
convertible, cumulative preferred shares" and 4,600,000 shares are designated as
"8 5/8% series C cumulative redeemable preferred shares of beneficial interest."

     Under the Equity Office declaration of trust, the Equity Office board of
trustees has the authority to issue authorized but unissued common shares or
preferred shares in one or more classes or series without shareholder approval.
The Equity Office board of trustees also is authorized to reclassify authorized
but unissued common shares into preferred shares, and authorized but unissued
preferred shares into common shares, without shareholder approval. Subject to an
express provision to the contrary in the terms of any class or series of
authorized shares, under the Equity Office declaration of trust the board of
trustees also has the power to divide or combine the outstanding shares of any
class or series, without shareholder approval. Notwithstanding the foregoing,
under the articles supplementary establishing the outstanding series A, series B
and series C preferred shares of Equity Office, the board of trustees may not
authorize, create or increase the authorized amount of any class or series of
shares ranking before the outstanding series A, series B and series C shares
with respect to the payment of distributions or upon liquidation, without the
approval of holders of two thirds of the outstanding shares of these series of
preferred shares voting together as a single class.

     At       , 2000, there were issued and outstanding           Equity Office
common shares,           Equity Office series A preferred shares,
          Equity Office series B convertible, cumulative preferred shares and
          Equity Office series C preferred shares.

     The authorized stock of Cornerstone consists of 250,000,000 shares of
common stock, without par value, and 65,000,000 shares of preferred stock,
without par value, of which 3,030,303 shares are designated as "7% cumulative
convertible preferred stock" and 344,828 shares are designated as "redeemable
preferred stock."

     Under the Cornerstone articles of incorporation, the Cornerstone board of
directors has the authority to issue authorized but unissued common shares or
preferred shares in one or more series without stockholder approval.

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<PAGE>   87

     At       , 2000, there were issued and outstanding           shares of
Cornerstone common stock and 3,030,303 shares of Cornerstone 7% cumulative
convertible preferred stock.

VOTING RIGHTS

     Each holder of Equity Office common shares is entitled to one vote per
share and to the same and identical voting rights as other holders of Equity
Office common shares. Holders of Equity Office common shares do not have
cumulative voting rights. Except as provided in articles supplementary
establishing a series of Equity Office preferred shares or required by law,
holders of Equity Office preferred shares do not have any voting rights. For a
description of the voting rights of holders of outstanding series A, series B
and series C preferred shares, see "-- Authorized Shares;" "-- Number of
Trustees/Directors; Removal of Trustees/Directors; Vacancies;" and "-- Amendment
of the Declaration of Trust and Articles of Incorporation."

     Each holder of Cornerstone common stock is entitled to one vote per share
and to the same and identical voting rights as other holders of Cornerstone
common stock. Holders of Cornerstone common stock do not have cumulative voting
rights. Except as provided by law or in the certificate of designations creating
a series of Cornerstone preferred stock, holders of Cornerstone preferred stock
do not have any voting rights.

CLASSIFICATION OF THE BOARD

     The Maryland real estate investment trust law permits a Maryland real
estate investment trust's board of trustees to be divided into up to three
classes with staggered terms of office. The Equity Office declaration of trust
divides the Equity Office board of trustees into three classes, with classes
being elected to three-year terms on a rotating basis.

     The Nevada General Corporation Law permits a Nevada corporation to divide
its board of directors into up to four classes with staggered terms of office.
However, Cornerstone does not have a classified board of directors. Accordingly,
its directors serve for one-year terms and are elected annually.

NUMBER OF TRUSTEES/DIRECTORS; REMOVAL OF TRUSTEES/DIRECTORS; VACANCIES

     The Equity Office declaration of trust provides for a minimum of nine and a
maximum of 15 trustees, with the number of trustees within this range
established by a majority vote of the board of trustees as provided in the
Equity Office bylaws. Under the Equity Office bylaws, except during the period
in which a vacancy exists, at least two-thirds of the Equity Office trustees
must be persons who are not executive officers of Equity Office or persons
affiliated with Samuel Zell or his affiliates.

     Under the Equity Office declaration of trust, subject to the rights of one
or more classes or series of preferred shares to elect one or more trustees, a
trustee may be removed at any time, only with cause, at a meeting of the
shareholders by the affirmative vote of the holders of not less than a majority
of the shares then outstanding and entitled to vote generally in the election of
trustees.

     Under the Equity Office declaration of trust, subject to the rights of
holders of any class or series of preferred shares, each vacancy on the board of
trustees, including a vacancy resulting from an increase in the number of
trustees, may be filled only by the affirmative vote of a majority of the
remaining trustees in office, even if the remaining trustees do not constitute a
quorum. Any trustee elected to fill a vacancy will hold office for the remainder
of the full term of the class of trustees in which the vacancy occurred and
until a successor is elected and qualifies.

     The holders of the outstanding shares of series A, series B and series C
preferred shares of Equity Office are entitled, voting together as a single
class, to elect a total of two trustees to the Equity Office board of trustees
at any time distributions on the preferred shares were in arrears for six or
more quarterly periods.

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     Cornerstone's articles of incorporation provide for a minimum of three and
a maximum of eighteen members of Cornerstone's board of directors. At present,
Cornerstone's board stands at fourteen members. The size of the board may be
increased or decreased by action of the board of directors and, during certain
periods set forth in Cornerstone's bylaws, the approval of the board affairs
committee of Cornerstone's board is required to effect any such change.
Cornerstone's bylaws also require that: (a) until December 11, 2001,
Cornerstone's board of directors shall nominate William Wilson III and two
individuals designated by Mr. Wilson for election to the board; and (b) until
the date that PGGM and its affiliated entities own, collectively, less than 5%
of Cornerstone's issued and outstanding common stock, Cornerstone's board is
required to nominate two persons designated by PGGM for election as directors.

     At least one of the two persons designated by Mr. Wilson must satisfy the
criteria for independent directors described in the New York Stock Exchange,
Inc. Listed Company Manual and must satisfy the criteria for non-employee
directors described in rule 16b-3(b)(3)(i) promulgated by the SEC under the
Securities Exchange Act of 1934.

     Directors may be removed, with or without cause, at any time by the vote or
written consent of stockholders of record holding at least two-thirds of the
shares of Cornerstone's issued and outstanding common stock.

     Vacancies on Cornerstone's board may be filled by the affirmative vote or
written consent of stockholders of record holding at least a majority of the
shares of Cornerstone's issued and outstanding common stock or by vote or
written consent of Cornerstone's board, whether or not sufficient directors are
in office to constitute a quorum. During the periods referred to above in which
either Mr. Wilson or PGGM has a right to designate persons for nomination to the
board of directors, if a vacancy on Cornerstone's board is caused by the absence
of any such person, Cornerstone's board is required to fill the vacancy with a
person designated by Mr. Wilson or PGGM, as the case may be. Any director
elected to fill a vacancy on Cornerstone's board will hold office for the
remainder of the full term of the absent director.

     If dividends payable on Cornerstone's 7% cumulative convertible preferred
stock are in arrears for an aggregate number of days equal to six calendar
quarters or more, whether or not consecutive, the holders of such stock have the
right to elect two members of Cornerstone's board of directors at the next
annual meeting of shareholders and at each subsequent meeting of shareholders
until such arrearages have been paid or set apart for payment.

LIMITATION OF TRUSTEE/DIRECTOR AND OFFICER LIABILITY

     Under Maryland law, the declaration of trust of a Maryland real estate
investment trust may include a provision expanding or limiting the liability of
its trustees and officers to the trust or its shareholders for money damages but
may not include a provision which restricts or limits the liability of its
trustee or officers to the trust or its shareholders to the extent that:

     - it is proved that the person actually received an improper benefit or
       profit in money, property or services, for the amount of the benefit or
       profit in money, property or services actually received; or

     - a judgment or other final adjudication adverse to the person is entered
       in a proceeding based on a finding that the person's action, or failure
       to act, was the result of active and deliberate dishonesty and was
       material to the cause of action adjudicated in the proceeding.

     The Equity Office declaration of trust provides that, to the maximum extent
that Maryland law, no trustee or officer of Equity Office shall be liable to
Equity Office or to any shareholder for money damages.

     Nevada law allows a corporation to provide in its articles of incorporation
that the personal liability of the corporation's directors to its stockholders
or the corporation is limited, but such a provision may not extend to
intentional misconduct, fraud or a knowing violation of law or payments of
distributions to the

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corporation's shareholders which are not allowed by Nevada law. Cornerstone's
articles of incorporation do not contain any such provision limiting director
liability.

INDEMNIFICATION

     Maryland law generally permits a Maryland real estate investment trust to
indemnify any person made a party to any threatened, pending or completed
action, suit or proceeding by reason of the fact that the person is or was a
trustee, officer, employee or agent of the trust or any predecessor entity, or
is or was serving at the request of the trust or predecessor entity as a
director, officer, partner, trustee, employee or agent of another corporation,
partnership, joint venture, trust, other enterprise or employee benefit plan,
unless it is established that:

     - the act or omission was material to the matter giving rise to the
       proceeding and either was committed in bad faith or was the result of
       active and deliberate dishonesty;

     - the person actually received an improper personal benefit in money,
       property or services; or

     - in the case of any criminal proceeding, the person had reasonable cause
       to believe that the act or omission was unlawful.

     Under Maryland law, indemnity may be provided against judgments, penalties,
fines, settlements and reasonable expenses actually incurred by the person in
connection with the proceeding. The indemnification may be provided, however,
only if authorized for a specific proceeding after a determination has been made
that indemnification is permissible in the circumstances because the person met
the applicable standard of conduct. This determination is required to be made:

     - by the board of trustees by a majority vote of a quorum consisting of
       trustees not, at the time, parties to the proceeding;

     - if such quorum cannot be obtained, then by a majority vote of a committee
       of the board consisting solely of two or more trustees not, at the time,
       parties to the proceeding;

     - by special legal counsel; or

     - by the shareholders.

     If the proceeding is one by or in the right of the trust, indemnification
may not be provided as to any proceeding in which the person is found liable to
the trust.

     A Maryland real estate investment trust may pay, before final disposition,
the expenses, including attorneys' fees, incurred by a trustee, officer,
employee or agent in defending a proceeding. Under Maryland law, expenses may be
advanced to a trustee or officer when the trustee or officer gives an
undertaking to the trust to repay the amounts advanced if it is ultimately
determined that he or she is not entitled to indemnification. Maryland law does
not require that the undertaking be secured and the undertaking may be accepted
without reference to the financial ability of the trustee or officer to repay
the advance. A Maryland trust is required to indemnify any trustee who has been
successful, on the merits or otherwise, in defense of a proceeding for
reasonable expenses. The determination as to reasonableness of expenses is
required to be made in the same manner as required for indemnification.

     Under Maryland law, the indemnification and advancement of expenses
provided by statute are not exclusive of any other rights to which a person
seeking indemnification or advancement of expenses may be entitled under any
bylaw, agreement, vote of shareholders, vote of trustees or otherwise.

     Under the Equity Office bylaws, Equity Office is required to indemnify and
advance expenses to present and former trustees and officers, and may indemnify
employees and agents, of Equity Office and its predecessors. The Equity Office
bylaws also provide for mandatory indemnification and advancement of expenses to
present and former shareholders of Equity Office or its predecessors made a
party to a proceeding by reason of such status.

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     The limited partnership agreement of EOP Operating Limited Partnership also
provides for indemnification of Equity Office and its officers and trustees to
the same extent that indemnification is provided to officers and trustees of
Equity Office in its declaration of trust, and limits the liability of Equity
Office and its officers and trustees to EOP Partnership and its respective
partners to the same extent that the Equity Office declaration of trust limits
the liability of the officers and trustees of Equity Office to Equity Office and
its shareholders.

     Section 78.7502 of Chapter 78 of the Nevada Revised Statutes allows
Cornerstone to indemnify any person who was or is a party or is threatened to be
made a party to any threatened, pending or completed action or suit or
proceeding, whether civil, criminal, administrative or investigative, other than
an action by or in the right of the corporation, by reason of the fact that he
or she is or was a director, officer, employee or agent of the corporation or is
or was serving at the request of the corporation as a director, officer,
employee or agent of another corporation or other enterprise, against expenses,
including attorneys' fees, and against judgments, fines and amounts paid in
settlement actually and reasonably incurred by him or her in connection with
such action, suit or proceeding if:

     - he or she acted in good faith and in a manner he or she reasonably
       believed to be in, or not opposed to, the best interests of the
       corporation; and

     - with respect to any criminal action or proceeding, he or she had no
       reasonable cause to believe his or her conduct was unlawful.

     The termination of any action, suit or proceeding by judgment, order,
settlement or conviction or upon a plea of nolo contendere or its equivalent
does not, of itself, create a presumption that the person did not act in good
faith or in a manner which he or she reasonably believed to be in, or not
opposed to, the best interests of the corporation or that, with respect to any
criminal action or proceeding, he or she had reasonable cause to believe his or
her actions were unlawful.

     Section 78.7502 of the Nevada Revised Statutes allows Cornerstone to
indemnify any person who was or is a party or is threatened to be made a party
to any threatened, pending or completed action or suit by or in the right of the
corporation to procure a judgment in its favor by reason of the fact that such
person acted in any of the capacities described above against expenses,
including attorneys' fees, actually and reasonably incurred by him or her in
connection with the defense or settlement of such action or suit if he or she
acted under similar standards to those described above, except that no
indemnification may be made in respect of:

     - any claim, issue or matter as to which such person shall have been
       adjudged to be liable to the corporation; or

     - amounts paid in settlement to the corporation, unless and only to the
       extent that the court in which such action or suit was brought determines
       that, despite the adjudication of liability, such person is fairly and
       reasonably entitled to indemnity for such expenses as the court deems
       proper.

     Section 78.7502 of the Nevada Revised Statutes further provides that to the
extent a director or officer of a corporation has been successful in the defense
of any action, suit or proceeding referred to above or in the defense of any
claim, issue or matter therein, he or she shall be indemnified against expenses,
including attorneys' fees, actually and reasonably incurred by him or her in
connection therewith. Article IX of Cornerstone's bylaws provides for
indemnification of its officers and directors that is substantially identical in
scope to the foregoing discussion of the Nevada Revised Statutes.

     Section 78.751 of the Nevada Revised Statutes provides that any
indemnification provided for by Section 78.7502 of the Nevada Revised Statutes,
by court order or otherwise, shall not be deemed to be exclusive of any other
rights to which the indemnified party may be entitled and that the scope of
indemnification shall continue as to directors, officers, employees or agents
who have ceased to hold such positions, and to their heirs, executors and
administrators. Cornerstone's bylaws also provide, pursuant to Section 78.7502
of the Nevada Revised Statutes, that the expenses of officers and directors
incurred in defending any action, suit or proceeding, whether civil, criminal,
administrative or investigative, must be

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<PAGE>   91

paid by Cornerstone as they are incurred and in advance of the final disposition
of the action, suit or proceeding, provided that Cornerstone receives an
undertaking by or on behalf of the director or officer to repay such amounts
unless it shall ultimately be determined that he or she is entitled to be
indemnified by Cornerstone pursuant to Article IX of its bylaws.

     Finally, Section 78.752 of the Nevada Revised Statutes allows Cornerstone
to purchase and maintain insurance on behalf of any of Cornerstone's directors,
officers, employees or agents against any liability asserted against any of them
in any such capacity or arising out of their status as such, whether or not
Cornerstone would have the power to indemnify them against such liabilities
under Section 78.7502. Cornerstone has purchased this type of insurance in
customary amounts for its directors, officers, employees and agents.

DUTIES OF TRUSTEES AND DIRECTORS

     Recent Maryland legislation provides increased protection for Maryland real
estate investment trusts against unsolicited takeovers by affording greater
protection to a board of trustees with regard to actions taken in a takeover
context. The Maryland legislation provides that the duties of trustees will not
require them to:

     - accept, recommend, or respond to any proposal by a person seeking to
       acquire control;

     - authorize the real estate investment trust to redeem any rights under,
       modify, or render inapplicable a shareholder rights plan;

     - make a determination under the Maryland Business Combination Statute or
       the Control Share Acquisition Statute, as described below;

     - elect to be subject to any or all of the "elective provisions" described
       below; or

     - act or fail to act solely because of:

       -- the effect the act or failure to act may have on an acquisition or
          potential acquisition of control; or

      -- the amount or type of consideration that may be offered or paid to
         shareholders in an acquisition.

     The Maryland legislation also states the presumption that any act of a
trustee satisfies the required standard of conduct. In the case of a Maryland
corporation, these are a director's duties to act in good faith, in a manner
that is in the best interests of the corporation and with the care of an
ordinarily prudent person under similar circumstances. In the case of a Maryland
real estate investment trust, the standard of conduct is not explicitly provided
by statute. Under the Maryland legislation, an act of a trustee relating to or
affecting an acquisition or a potential acquisition of control may not be
subject to a higher duty or greater scrutiny than is applied to any other act of
a trustee. This provision creates a Maryland rule which is less exacting than
case law in many other jurisdictions which (a) imposes an enhanced level of
scrutiny when a board implements anti-takeover measures in a change of control
context; and (b) shifts the burden of proof to directors to show that the
defensive mechanism adopted by a board is reasonable in relation to the threat
posed.

     The Maryland legislation also provides that the duty of a trustee is only
enforceable by the trust or in the right of the trust. A shareholder suit to
enforce such duty, therefore, can only be brought derivatively.

     Partly in response to judicial decisions in Delaware in the takeover
context, Nevada's legislature enacted a statute providing that, in performing
their duties, directors of a corporation may consider factors other than the
short-term interests of the corporation's shareholders, including the interests
of the corporation's employees, suppliers, creditors and customers, the state
and national economy, the interests of the community and society, and the
long-term as well as short-term interests of the corporation and its
stockholders, including the possibility that these interests may be best served
by the corporation's continued independence.
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<PAGE>   92

     Like directors of Maryland corporations, directors of Nevada corporations
enjoy the benefit of a presumption that, in exercising their powers, directors
are acting in good faith, on an informed basis and with a view to the interests
of the corporation. The required standard of care is expressly stated in the
statute: good faith with a view to the interests of the corporation. Given,
however, the case law of other jurisdictions in the anti-takeover context, it is
not clear whether a Nevada court would impose a more exacting standard of care
than that stated in the statute.

MARYLAND ELECTIVE PROVISIONS

     Recent Maryland legislation allows publicly-held Maryland corporations and
Maryland real estate investment trusts to elect to be governed by all or any
part of new Maryland law provisions relating to extraordinary actions and
unsolicited takeovers. The election to be governed by one or more of these
provisions can be made by a Maryland real estate investment trust in its
declaration of trust or bylaws or by resolution adopted by the board of trustees
as long as the trust has at last three trustees who, at the time of electing to
be subject to the provisions are not:

     - officers or employees of the trust;

     - persons seeking to acquire control of the trust;

     - directors, officers, affiliates or associates of any person seeking to
       acquire control; or

     - nominated or designated as trustees by a person seeking to acquire
       control.

     Articles supplementary must be filed with the Maryland State Department of
Assessments and Taxation if a Maryland corporation trust or elects to be subject
to any or all of the provisions by board resolution or bylaw amendment.
Shareholder approval is not required for the filing of articles supplementary.

     The Maryland legislation provides that a trust can elect to be subject to
all or any portion of the following provisions notwithstanding any contrary
provisions contained in the trust's existing declaration of trust or bylaws:

     - Classified Board:  Equity Office already has a classified board;

     - Two-Thirds Shareholder Vote to Remove Trustees Only For Cause:  The
       shareholders may remove any trustee only by the affirmative vote of at
       least two thirds of all the votes entitled to be cast by the shareholders
       generally in the election of directors, but a trustee may not be removed
       without cause;

     - Size of Board Fixed by Vote of Board:  Equity Office already has this
       provision, subject to the range on the number of trustees contained in
       its declaration of trust;

     - Board Vacancies Filled by the Board for the Remaining Term:  Vacancies
       that result from an increase in the size of the board, or the death,
       resignation, or removal of a trustee, may be filled only by the
       affirmative vote of a majority of the remaining trustees even if they do
       not constitute a quorum. Trustees elected to fill vacancies shall hold
       office for the remainder of the full term of the class of directors in
       which the vacancy occurred (as opposed to until the next annual meeting
       of shareholders) and until a successor is elected and qualifies. Equity
       Office recently adopted this provision; and

     - Shareholder Calls of Special Meetings Only Upon the Written Request of
       Holders of at Least a Majority of All Votes Entitled to be Cast:  Equity
       Office already has this provision.

     Although Nevada's General Corporation Law contains no similar statute,
Cornerstone's bylaws and articles of incorporation address most of the issues
addressed by the Maryland legislation described above. Other than stockholder
calls for special meetings, Cornerstone's approach to the issues addressed by
the Maryland legislation is set forth in the above section entitled "Number of
Trustees/Directors; Removal of Trustees/Directors; Vacancies." Nevada law does
not provide for stockholder calls of special meetings.

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<PAGE>   93

However, Cornerstone's bylaws provide that holders of record of at least a
majority of the shares of Cornerstone's issued and outstanding common stock
which are entitled to vote at a special meeting may call such a meeting.

CALL OF SPECIAL MEETINGS OF SHAREHOLDERS

     The Equity Office bylaws provide that special meetings of shareholders may
be called by the chairman of the board, the president or one-third of the
trustees. Special meetings of shareholders also may be called by the holders of
shares entitled to cast not less than a majority of all the votes entitled to be
cast at a special meeting of shareholders.

     As noted above, Cornerstone's bylaws provide that holders of record of at
least a majority of the shares of Cornerstone's issued and outstanding common
stock which are entitled to vote at a special meeting may call a special meeting
of stockholders. In addition, Cornerstone's bylaws allow Cornerstone's board of
directors, Chairman of the Board, President or Secretary to call a special
meeting of the stockholders.

SHAREHOLDER ACTION BY WRITTEN CONSENT

     The Equity Office bylaws permit any action required or permitted to be
taken at a meeting of shareholders to be taken without a meeting if a consent in
writing, setting forth the action to be taken, is signed by shareholders
entitled to cast a sufficient number of votes to approve the matter.

     Nevada law and Cornerstone's bylaws also allow any action required or
allowed to be taken at a stockholder's meeting to be taken without a meeting,
notice or a vote of stockholders if a written consent to such action, setting
forth the action, is signed by holders of record of a majority, or if a higher
proportion would be required at a meeting of stockholders, then that higher
proportion, of the corporation's issued and outstanding shares of common stock.

ADVANCE NOTICE PROVISIONS FOR SHAREHOLDER NOMINATIONS AND SHAREHOLDER NEW
BUSINESS PROPOSALS

     The Equity Office bylaws require advance written notice for shareholders to
nominate a trustee or bring other business before a meeting of shareholders.

     For an annual meeting, a shareholder must deliver notice to the secretary
of Equity Office not later than the close of business on the 60th day nor
earlier than the close of business on the 90th day before the first anniversary
of the preceding year's annual meeting. However, if the date of the annual
meeting is advanced by more than 30 days or delayed by more than 60 days from
the applicable anniversary date of the prior year's annual meeting, or the
meeting is a special meeting of shareholders at which trustees will be elected,
notice by the shareholder must be given not earlier than the close of business
on the 90th day before the meeting and not later than the close of business on
the later of the 60th day before the meeting or the tenth day following the day
on which public announcement of the date of the meeting is first made by Equity
Office.

     The Equity Office bylaws contain detailed requirements for the contents of
shareholder notices of trustee nominations and new business.

     Neither the Cornerstone articles of incorporation nor its bylaws contain a
comparable provision.

AMENDMENT OF THE DECLARATION OF TRUST AND ARTICLES OF INCORPORATION

     Under Maryland law and the Equity Office declaration of trust, the
trustees, by a two-thirds vote, may at any time amend the declaration of trust
solely to enable Equity Office to qualify as a REIT under the Internal Revenue
Code or as a real estate investment trust under Maryland law, without action by
Equity Office shareholders. The Equity Office board of trustees also may amend
the declaration of trust to set the terms of one or more series of preferred
shares, without action by holders of Equity Office common shares. Other
amendments to the declaration of trust must first be declared advisable by the
board of trustees and

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<PAGE>   94

thereafter must be approved by shareholders by the affirmative vote of not less
than two-thirds of all votes entitled to be cast, or, in the case of amendments
to the declaration of trust in connection with mergers and other specified
business combinations or that involve an increase or decrease in the number of
authorized common shares or preferred shares, not less than a majority of all
votes entitled to be cast.

     Under the articles supplementary for each series of outstanding preferred
shares of Equity Office, the approval of holders of each preferred series by a
two-thirds class vote would be required for:

     - the authorization, creation or increase in the authorized or issued
       amount of any class or series of Equity Office shares ranking before the
       outstanding preferred series as to distributions or upon liquidation; or

     - any amendment, alteration or repeal of provisions of the declaration of
       trust, whether by merger, consolidation, or otherwise, so as to
       materially and adversely affect any right, preferences, privileges or
       voting power of the outstanding series of preferred shares.

     General amendments to Cornerstone's articles of incorporation are governed
by the provisions of the Nevada General Corporation Law. Cornerstone's articles
expressly provide, however, that Cornerstone's board of directors may determine
the rights, preferences, privileges, designations and other characteristics of
classes and series of Cornerstone's preferred stock. Under the Nevada General
Corporation Law, general amendments to Cornerstone's articles of incorporation
require Cornerstone's board of directors to recommend the amendment to
Cornerstone's stockholders and for holders of record of at least a majority of
the shares of Cornerstone's issued and outstanding common stock which are
entitled to vote to approve such amendment. Nevada's General Corporation Law
provides holders of classes or series of Cornerstone's preferred stock a right
to vote as a class or series on amendments to Cornerstone's articles of
incorporation, regardless of limitations on the voting rights of such class or
series, if the amendment would alter or change any preference or relative or
other right given to such a class or series.

     The certificate of designations of the only series of Cornerstone's
preferred stock of which shares are issued and outstanding, the 7% cumulative
convertible preferred stock, provides that the consent or affirmative vote of
holders of at least two-thirds of the shares of the holders thereof shall be
required to:

          (a) issue or increase the authorized amount of any class or series of
     shares ranking prior with respect to payment of dividends or distributions
     of amounts upon liquidation; or

          (b) amend, alter or repeal any provision of Cornerstone's articles of
     incorporation that would materially and adversely affect any power,
     preference or special right of the series.

     Notwithstanding (b) above, Nevada's General Corporation Law would require
the approval of the record holders of a majority of issued and outstanding
shares of the 7% cumulative convertible preferred stock that are entitled to
vote, to amend Cornerstone's articles of incorporation, whether or not the
proposed amendment would "materially" alter or change any preference or any
relative or other right given to such stock.

AMENDMENT OF THE BYLAWS

     The Equity Office bylaws provide that the power to amend, repeal or adopt
new bylaws is vested exclusively with the board of trustees, except that any
amendments by the board of trustees to the bylaw provisions relating to meetings
of shareholders, the minimum and maximum number of trustees, the requirement
that at least two-thirds of the trustees must be persons who are not executive
officers of Equity Office or persons affiliated with Samuel Zell or his
affiliates are subject to the approval of shareholders by vote of a majority of
the votes cast.

     Amendment of Cornerstone's bylaws is governed by the terms of the bylaws.
The bylaws provide that amendments may be adopted either by holders of record of
a majority of the corporation's issued and outstanding shares of common stock or
by Cornerstone's board. However, until December 16, 2001, amendments to Sections
3.02 (number of directors and term), 3.03 (identity of members of the board of
directors), 3.04 (relating to the position of chairman of the board), 3.07
(filling vacancies on the board of
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<PAGE>   95

directors), 3.09 (relating to committees of the board of directors), and
10.01(b) (relating to restrictions on amendment of the bylaws), require the
consent of a majority of the members of the entire board and the unanimous
approval of the board affairs committee of the board of directors. In addition,
until the earlier of (a) the date on which PGGM or Dutch Institutional Holding
Company or their respective affiliates own, collectively, less than 25% of
Cornerstone's issued and outstanding common stock and (b) October 31, 2002,
amendments to Sections 3.02, 3.03, 3.07, 3.09 or 10.01(c) (relating to
restrictions on amendments of the bylaws) of the bylaws require the consent of a
majority of the members of the entire board and the unanimous approval of the
board affairs committee of the board of directors. Lastly, until PGGM and its
affiliated entities own, collectively, less than 5% of the issued and
outstanding shares of Cornerstone's common stock, amendments to Section 3.03(b)
(relating to PGGM's right to designate persons for nomination to Cornerstone's
board of directors) and 10.01(c) also require the consent of a majority of the
members of the entire board and the unanimous approval of the board affairs
committee of the board of directors.

MERGERS, CONSOLIDATIONS AND SALES OF ASSETS

     Under Maryland law, a merger involving a Maryland real estate investment
trust generally requires approval by the affirmative vote of not less than
two-thirds of all votes entitled to be cast on the matter, unless the
declaration of trust specifies a greater or lesser percentage, but not less than
a majority. No shareholder approval is required for parent-90% owned subsidiary
mergers or by shareholders of a Maryland successor trust if the merger does not
reclassify or change the outstanding shares or otherwise amend the declaration
of trust and the number of shares to be issued in the merger is not more than
20% of the number of its shares of the same class or series outstanding
immediately before the merger is completed. The Equity Office declaration of
trust specifies that the affirmative vote of shareholders by not less than a
majority of all votes entitled to be cast is required to approve mergers for
which a shareholder vote is required under Maryland law.

     Maryland law does not address the requirements for the approval by
shareholders of a consolidation or sale of all or substantially all of the
assets of a real estate investment trust. However, the Equity Office declaration
of trust requires that a majority of the Equity Office shares entitled to vote
on the matter must approve a consolidation of Equity Office into one or more
other entities or the sale of all or substantially all of the assets of Equity
Office outside the ordinary course of business. Under the Equity Office
declaration of trust, the mortgage, pledge or other creation of a security
interest in any or all of the assets of Equity Office, whether or not in the
ordinary course of business, as well as the sale of all or substantially all of
the assets of Equity Office to a majority owned subsidiary or as a distribution
to shareholders is not deemed to be a sale requiring shareholder approval.

     Mergers and share exchanges involving Nevada corporations are governed by
Chapter 92A of the Nevada Revised Statutes, which we refer to as the "Merger
Statute." Under the Merger Statute, Cornerstone may approve a plan of merger or
exchange of shares if (a) Cornerstone's board of directors recommends the plan
to the stockholders and (b) holders of record of a majority of Cornerstone's
common stock entitled to vote approve the plan. Approval of a merger by
Cornerstone's stockholders is not required if: (x) Cornerstone is to be the
surviving entity in a merger; (y) Cornerstone's articles of incorporation after
the merger will be the same as its articles of incorporation before the merger;
and (z) the number of shares of Cornerstone's common stock outstanding
immediately after the merger, plus the number of such shares issued as a result
of the merger, either by the conversion of securities issued pursuant to the
merger or the exercise of rights and warrants issued pursuant to the merger,
will not exceed the total number of shares of Cornerstone's common stock
outstanding immediately before the merger by more than 20%.

     Nevada's General Corporation Law requires the consent of the holders of a
majority of the shares of Cornerstone's common stock entitled to vote for a
sale, lease, or exchange of all of its property and assets.

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DISSOLUTION OF EQUITY OFFICE OR CORNERSTONE; TERMINATION OF REIT STATUS

     The Equity Office declaration of trust permits the termination of the
existence of Equity Office if approved by the affirmative vote of the holders of
not less two-thirds of the outstanding Equity Office shares entitled to vote on
the matter. In addition, the board of trustees may terminate the status of
Equity Office as a REIT under the Internal Revenue Code for any taxable year
without a vote of the holders of Equity Office common or preferred shares.

     Under Nevada's General Corporation Law, Cornerstone may be dissolved if its
board of directors recommends dissolution to the stockholders and holders of a
majority of the shares of Cornerstone's outstanding common stock entitled to
vote approve such dissolution.

BUSINESS COMBINATIONS WITH INTERESTED SHAREHOLDERS

     The Maryland Business Combination Statute provides that, unless exempted, a
Maryland real estate investment trust may not engage in business combinations,
including mergers, dispositions of 10% or more of its assets, issuances of
shares and other specified transactions, with an "interested shareholder" or its
affiliates, for five years after the most recent date on which the interested
shareholder became an interested shareholder. Thereafter, unless specified
"price criteria" and other standards are met or an exemption is available, a
business combination with an interested shareholder or its affiliates must be
recommended by the board of trustees and approved by (a) at least 80% of the
outstanding voting shares and (b) at least two-thirds of the outstanding voting
shares, other than voting shares held by the interested shareholder or any of
its affiliates. Under the statute, an "interested shareholder" generally is
defined to mean a person or group which owns beneficially, directly or
indirectly, 10% of more of the outstanding voting shares of the real estate
investment trust. These requirements do not apply to a business combination with
an interested shareholder or its affiliates if approved by the board of trustees
before the time the interested shareholder first became an interest shareholder.

     The Equity Office board of trustees has elected by resolution to exempt
form the provisions of the Maryland Business Combination Statute any business
combination with any person. However, this resolution, by its terms, may be
altered or repealed at any time, in whole or in part, by the board of trustees.

     Nevada's Business Combinations Act prohibits combinations between a Nevada
corporation and an interested stockholder of the corporation for three years
after the stockholder becomes interested unless the combination or acquisition
of shares by the interested stockholder on the date the stockholder became an
interested stockholder was approved by the corporation's board of directors
before that date. An interested stockholder is: (a) a person who, directly or
indirectly, has the ability to control the voting of 10% of the voting stock of
the corporation; or (b) an affiliate or associate of the corporation who, at any
time within three years before the date in question, satisfied the definition in
(a). A combination is broadly defined and includes mergers, dispositions of 5%
or more of its assets, issuances of shares and other specified transactions.

     Although a Nevada corporation may, in its articles of incorporation, opt
out of the Business Combinations Act, Cornerstone has not done so.

CONTROL SHARE ACQUISITIONS

     The Maryland Control Share Act provides that shares of a Maryland real
estate investment trust that are acquired in a "control share acquisition,"
which is defined as the acquisition of shares comprising one-fifth, one-third or
a majority of all voting shares, have no voting rights except:

     - if approved by shareholders by the affirmative vote of two-thirds of all
       the votes entitled to be cast on the matter, excluding all "interested
       shares;" or

     - if the acquisition of the shares has been approved or exempted at any
       time before the acquisition of the shares.

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The Maryland Control Share Act is applicable to a publicly-traded Maryland real
estate investment trust unless its charter or bylaws specifically provides that
it shall be inapplicable.

     The Equity Office bylaws provide that the Maryland Control Share Act shall
not apply to any acquisition by any person of shares of Equity Office. Any
amendment to this provision by the board of trustees would require the approval
of shareholders by a vote of a majority of the votes cast. See "Amendment of the
Bylaws."

     Nevada's Control Shares Act restricts the voting rights of shares of stock
acquired by a stockholder of a Nevada corporation governed by the act if the
acquisition of such shares would provide such stockholder a controlling interest
in the corporation. A controlling interest is ownership of one-fifth or more but
less than one-third, one-third or more but less than a majority, or a majority
or more of the corporation's shares of stock entitled to vote generally in the
election of directors. Thus, the act applies to acquisitions of shares as a
stockholder moves from one level of control to the next. Only the shares
acquired within 90 days immediately preceding the date when the stockholder
acquires the relevant controlling interest are subject to the limitation on
voting rights.

     The Control Shares Act applies only to Nevada corporations (a) with 200 or
more stockholders of record, at least 100 of whom have addresses in Nevada
appearing in the corporation's stock ledger and (b) doing business in Nevada
directly or through an affiliated corporation. Cornerstone does not currently
have 100 stockholders of record who have addresses in Nevada appearing in
Cornerstone's stock ledger and Cornerstone does not do business in Nevada
directly or through an affiliated corporation. Moreover, the provisions of the
Control Shares Act do not apply to a Nevada corporation if the corporation's
articles of incorporation or bylaws in effect on the tenth day following the
acquisition of a controlling interest by a stockholder provide that such
provisions do not apply to the corporation or to an acquisition of a controlling
interest specifically by types of existing or future stockholders, whether or
not identified.

     Neither Cornerstone's bylaws nor its articles of incorporation contain such
an opt-out provision.

OTHER CONSTITUENCIES

     Recent Maryland legislation expressly codifies the authority of Maryland
real estate investment trusts to include in their declaration of trusts a
provision that allows the board of trustees to consider the effect of a
potential acquisition of control on shareholders, employees, suppliers,
customers, creditors and communities in which offices or other establishments of
the trust are located. The Equity Office declaration of trust does not include a
provision of this type. The Maryland legislation also states, however, that the
inclusion or omission of such a provision in the declaration of trust of a
Maryland real estate investment trust allowing the board of trustees to consider
the effect of a potential acquisition of control on the foregoing constituencies
does not create an inference concerning factors that may be considered by the
board of trustees regarding a potential acquisition of control.

     Nevada law contains a provision similar to the Maryland legislation
described above. The Nevada provision is discussed above under the heading
"Duties of Trustees and Directors."

DISSENTERS' RIGHTS

     Maryland law applicable to Maryland real estate investment trusts provides
dissenters' rights for any shareholder who objects to a merger to the same
extent as a Maryland corporation's stockholders would enjoy such rights. The
shareholder has the right to demand and receive payment of the fair value of his
or her shares, unless:

     - the shares are listed on a national securities exchange or are designated
       as a national market security on the interdealer quotation system of the
       National Association of Securities Dealers, Inc. on the record date for
       determining shareholders entitled to vote on the merger; or

     - the shares are those of a successor entity, as long as the merger does
       not alter the contract rights of the shares as expressly set forth in the
       declaration of trust and the shares are converted in whole or

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       in part in the merger into stock, including preferred stock, of the
       successor entity or cash, scrip, or other rights or interests arising out
       of the treatment of fractional shares.

     Nevada law regarding dissenter's rights is set forth in the Merger Statute.
Under the Merger Statute, a shareholder of a Nevada corporation may dissent with
respect to specified corporation actions and, as a result, receive fair value
for the stockholder's shares. Those actions to which dissenter's rights apply
are: (a) a merger of the corporation if shareholder approval of the merger is
required by the Merger Statute or the corporation's articles of incorporation
and the stockholder is entitled to vote thereon; (b) merger of the corporation
into a parent corporation that owns at least ninety percent of each class of the
outstanding shares of the corporation if the merger is effected through Section
92A.180 of the Nevada Revised Statutes; (c) consummation of a plan of exchange
in which the corporation's shares will be acquired if the stockholder is
entitled to vote thereon; and (d) other transactions to which the corporation's
articles of incorporation attach the right to dissent and obtain fair payment
for dissenter's shares.

     Dissenter's rights are not available, however, if the corporation is listed
on a national securities exchange, as Cornerstone is, unless (a) the
corporation's articles of incorporation provide otherwise or (b) the
stockholders are required under the plan of merger or exchange to accept
anything for their shares other than cash, stock or stock and cash in lieu of
fractional shares of stock of the surviving corporation or any other entity
whose stock was listed on a national securities exchange or NASDAQ or which had
at least 2,000 holders of record of such entity's stock, or any combination of
the above. As used in this paragraph, "stock" refers to shares of stock in a
corporation, membership in a nonprofit corporation, the interest of a member in
a limited-liability company or a beneficial owner of a business trust, or the
partnership interest of a partner in a general or limited partnership.

DISTRIBUTIONS

     The Equity Office declaration of trust provides that the trustees will
endeavor to declare and pay distributions as necessary for Equity Office to
qualify as a REIT under the Internal Revenue Code. However, shareholders do not
have any right to a distribution unless and until authorized and declared by the
board of trustees. Distributions may not be paid on the Equity Office common
shares unless all accrued but unpaid distributions on each outstanding series of
preferred shares of Equity Office have been declared and paid or set apart for
payment. Payments of distributions by Equity Office are not limited by any rules
concerning the capital or surplus of Equity Office or the par value of the
Equity Office shares.

     Nevada's General Corporation Law provides that the board of directors may
not make a distribution of money or property to its stockholders if such
distribution would prevent the corporation from paying its debts as they became
due in the ordinary course of business or, unless and to the extent specifically
allowed by the corporation's articles of incorporation, if after the
distribution, the corporation's total assets would be less than the sum of its
total liabilities plus that amount that would be needed, if the corporation were
to be dissolved at the time of the distribution, to satisfy the preferential
rights to distributions upon dissolution of the corporation's shareholders whose
rights upon dissolution are superior to those receiving the distribution.

REIT OWNERSHIP LIMITATIONS

     For Equity Office to qualify as a REIT under the Internal Revenue Code, no
more than 50% in value of its outstanding shares of beneficial interest may be
owned, actually or constructively, by five or fewer "individuals," which, as
defined in the Internal Revenue Code for this purpose, includes certain
entities. In addition, if Equity Office, or an actual or constructive owner of
10% or more of the shares of Equity Office, owns, actually or constructively,
10% or more of a tenant of Equity Office, then the rent received by Equity
Office from that tenant will not be qualifying income for purposes of
determining whether Equity Office meets the requirements for qualification as a
REIT under the Internal Revenue Code. A REIT's shares also must be beneficially
owned by 100 or more persons.

     As a means of addressing these requirements, the Equity Office declaration
of trust provides that, subject to exceptions, no person may own, or be deemed
to own directly and/or by virtue of the attribution provisions of the Internal
Revenue Code, more than 9.9%, in value or number of shares, whichever is more
restrictive, of the issued and outstanding shares of any class or series of
shares. Under the Equity Office
                                       86
<PAGE>   99

declaration of trust, the board of trustees may increase the ownership limit
with respect to any class or series of shares. However, after giving effect to
this increase, five beneficial owners of common shares may not beneficially own
in the aggregate more than 49.5% of the outstanding common shares. In addition,
the Equity Office board of trustees is required to waive or modify the ownership
limit with respect to one or more persons who would not be treated as
"individuals" under the Internal Revenue Code if such person submits to the
Equity Office board of trustees specified information that demonstrates, to the
reasonable satisfaction of the board of trustees, that such ownership would not
jeopardize Equity Office's status as a REIT under the Internal Revenue Code. The
Equity Office declaration of trust further prohibits any person from
transferring any Equity Office common or preferred shares if the transfer would
result in shares of beneficial interest of Equity Office being owned by fewer
than 100 persons or otherwise would cause Equity Office not to qualify as a
REIT.

     If any transfer of shares or any other event would otherwise result in any
person violating the ownership limits, then the declaration of trust provides
that (a) the transfer will be void and of no force or effect with respect to the
prohibited transferee with respect to that number of shares that exceeds the
ownership limits and (b) the prohibited transferee would not acquire any right
or interest in the shares. The shares transferred in violation of the ownership
limit instead would be transferred automatically to a charitable trust, the
beneficiary of which would be a qualified charitable organization selected by
Equity Office.

     The trustee of the charitable trust would be required to sell the shares
transferred in violation of the ownership limit to a person or entity who could
own the shares without violating the ownership limit, and to distribute to the
prohibited transferee an amount equal to the lesser of the price paid by such
person for the shares transferred in violation of the ownership limit or the
sales proceeds received by the charitable trust for the shares. In the case of a
transfer for no consideration, such as a gift, the charitable trustee would be
required to sell the shares to a qualified person or entity and distribute to
the prohibited transferee an amount equal to the lesser of the fair market value
of the shares as of the date of the prohibited transfer or the sales proceeds
received by the charitable trust.

     Under its declaration of trust, Equity Office, or its designee, would have
the right to purchase the shares from the charitable trust at a price per share
equal to the lesser of (a) the price per share in the transaction that resulted
in the transfer of the shares to the charitable trust, or, in the case of a
devise or gift, the market price at the time of such devise or gift, and (b) the
market price of such shares on the date Equity Office, or its designee, were to
agree to purchase the shares. Any proceeds derived from the sale of the shares
in excess of the amount distributed to the prohibited transferee under these
provisions would be distributed to the beneficiary of the charitable trust.

     The charitable trustee will have the sole right to vote the shares that it
holds, and any distributions paid on shares held by the charitable trustee would
be paid to the beneficiary of the charitable trust.

     If the transfer to the charitable trust of the shares that were transferred
in violation of the ownership limit is not automatically effective for any
reason, then the transfer to that resulted in the violation of the ownership
limit would be void.

     All persons or entities who own, directly or by virtue of the attribution
provisions of the Internal Revenue Code, more than 5%, or such other percentage
between 1/2 of 1% and 5% as provided in applicable rules and regulations under
the Internal Revenue Code, of the lesser of the number or value of the
outstanding Equity Office shares must give a written notice to Equity Office by
January 30 of each year stating the name and address of such owner, the number
of Equity Office shares owned and a description of the manner in which such
Equity Office shares are held. In addition, a holder of record of Equity Office
shares subject to the foregoing requirement who holds its Equity Office shares
as nominee for another person or entity which is required to include in gross
income the dividends received on such shares must also give notice of the name
and address of such person or entity and the number of Equity Office shares of
such person or entity with respect to which such holder of record is nominee. In
addition, each record, beneficial and constructive holder of Equity Office
shares is required, upon demand of Equity Office, to disclose to Equity Office
in writing any information with respect to the direct, indirect and
                                       87
<PAGE>   100

constructive ownership of Equity Office shares as the Equity Office board of
trustees deems necessary to comply with the provisions of the Internal Revenue
Code applicable to REITs, to comply with the requirements of any taxing
authority or governmental agency or to determine any such compliance.

     As part of the merger, the Equity Office declaration of trust will be
amended to add an additional limitation on the ownership by non-U.S. persons of
Equity Office common shares and preferred shares issued by Equity Office in the
future. See "Amendment to Equity Office Declaration of Trust to be Effected as
Part of the Merger." This additional limitation would result in adverse
consequences for a non-U.S. person whose purchase of Equity Office common shares
or any preferred shares of Equity Office issued in the future caused the foreign
ownership limit to be exceeded and, therefore, may deter additional purchases of
Equity Office shares by non-U.S. persons.

     The foregoing restrictions on ownership and transferability would not apply
if the Equity Office board of trustees were to determine that it is no longer in
the best interests of Equity Office to attempt to qualify, or to continue to
qualify, as a REIT under the Internal Revenue Code.

     To help ensure that Cornerstone continues to qualify as a REIT, Article 8
of Cornerstone's articles of incorporation provides that shares of Cornerstone's
capital stock shall not be transferred to any person if such transfer would
cause such person to become the direct or indirect owner of more than 6% of the
value of the outstanding shares of Cornerstone's capital stock. Shares of stock
acquired in excess of this ownership limit shall be deemed to be transferred to
Cornerstone as trustee for the benefit of the person to whom the person who
acquired the excess shares later transfers such shares. In addition, excess
shares shall be deemed to have been offered for sale to Cornerstone or its
designee at their "fair market value" for a 90-day period. Article 8 further
provides that a person who knowingly violates the ownership limit must indemnify
Cornerstone and its stockholders for losses if such violation causes Cornerstone
either to fail to qualify as a REIT or to be subject to personal holding company
taxes. The affirmative vote of the holders of at least 80% of the outstanding
shares of Cornerstone common stock is required to alter, amend, repeal or adopt
any provision inconsistent with such restrictions.

     Cornerstone's articles of incorporation also contain provisions designed to
assist it in becoming a "domestically controlled REIT" under the Code. Article 9
of Cornerstone's articles of incorporation provides that shares of Cornerstone's
stock may not be transferred to any non-U.S. person if such transfer would cause
such non-U.S. person to become the direct or indirect owner of more than 3% of
the value of the outstanding shares of any class of capital stock of
Cornerstone. Shares of stock acquired in excess of the non-U.S. ownership limit
are deemed to be transferred to Cornerstone as trustee for the benefit of the
person to whom the non-U.S. person who acquired the excess shares later
transfers such shares. In addition, excess shares are deemed to have been
offered for sale to Cornerstone or its designee at their "fair market value" for
a 90-day period. Article 9 does not apply, however, to transfers by any U.S.
person by operation of law or pursuant to death, a merger or sale of
substantially all of the assets of such person, bankruptcy or pledges of
security.

     As used in the preceding paragraph: "non-U.S. person" means (a) a
nonresident alien individual, as defined in the Internal Revenue Code, (b) a
foreign corporation, foreign partnership, foreign trust, foreign estate, foreign
government, and any other organization or entity which is not organized under
the laws of the United States or a State, and (c) any other person or entity
treated as a "foreign person" under regulations promulgated under the Internal
Revenue Code. "U.S. person" means any person other than a non-U.S. person.

                                 LEGAL MATTERS

     The validity of the Equity Office common shares offered by this joint proxy
statement/prospectus will be passed upon for Equity Office by Hogan & Hartson
L.L.P., Washington, D.C. and New York, NY.

     The qualification of the merger as a reorganization within Section 368(a)
of the Internal Revenue Code, and the qualification of Equity Office as a REIT
for federal income tax purposes will be passed

                                       88
<PAGE>   101

upon for Equity Office by Hogan & Hartson L.L.P., Washington, D.C. and New York,
NY, special tax counsel for Equity Office.

     The qualification of the merger as a reorganization within Section 368(a)
of the Internal Revenue Code, and the qualification of Cornerstone as a REIT for
federal income tax purposes will be passed upon for Cornerstone by King &
Spalding, Atlanta, Georgia and New York, New York, special tax counsel for
Cornerstone.

                                    EXPERTS

     The financial statements of Cornerstone incorporated in this joint proxy
statement/prospectus by reference to its Annual Report on Form 10-K for the year
ended December 31, 1999, have been so incorporated in reliance on the report of
PricewaterhouseCoopers LLP, independent accountants, given on the authority of
said firm as experts in auditing and accounting.

     Ernst & Young LLP, independent auditors, have audited the consolidated
balance sheets of Equity Office as of December 31, 1999 and 1998, and the
related consolidated statements of operations, shareholders' equity and cash
flows of Equity Office for the years ended December 31, 1999 and 1998 and the
period from July 11, 1997 to December 31, 1997, and the related combined
statements of operations, owners' equity and cash flows of the Equity Office
Predecessors, as defined in Note 1 to the financial statements, for the period
from January 1, 1997 to July 10, 1997 and financial statement schedule listed in
the Index at Item 14(a) included in Equity Office's annual report on Form 10-K,
as set forth in their report, which is incorporated by reference in this joint
proxy statement/prospectus and elsewhere in the registration statement. The
Equity Office and Equity Office Predecessor financial statements and schedule
are incorporated by reference in reliance on Ernst & Young LLP's report, given
on their authority as experts in accounting and auditing.

                             SHAREHOLDER PROPOSALS

     Shareholder proposals intended to be presented at the 2001 annual meeting
of Equity Office shareholders must be received by the Secretary of Equity Office
no later than             , 2000, to be considered for inclusion in our proxy
statement relating to the 2001 meeting.

     To be considered for inclusion in the proxy statement relating to the
Equity Office 2001 meeting, shareholder proposals submitted outside the Rule
14(a)-8 processes must be received by the Secretary of Equity Office no later
than             , 2001 to be presented at the 2001 annual meeting of
shareholders, and discretionary authority may be used if untimely submitted.

     Due to the contemplated completion of the merger, Cornerstone does not
currently expect to hold a 2000 annual meeting of stockholders because
Cornerstone will be merged with and into Equity Office and it will cease to
exist as a separate legal entity. If the merger is not consummated and such a
meeting is held, to be eligible for inclusion in Cornerstone's proxy statement
and form of proxy relating to that meeting, proposals of stockholders intended
to be presented at the meeting must be received by Cornerstone within a
reasonable period of time after Cornerstone announces publicly the date of the
meeting and before Cornerstone mails its proxy statement to stockholders in
connection with the meeting.

                                 OTHER MATTERS

     As of the date of this joint proxy statement/prospectus, neither the board
of trustees of Equity Office nor the board of directors of Cornerstone knows of
any matters that will be presented for consideration at either special meeting
other than those described in this joint proxy statement/prospectus. If any
other matters properly come before either of the special meetings or any
adjournments or postponements of either of the special meetings, and are voted
upon, the enclosed proxies will confer discretionary authority on the
individuals named as proxies to vote the shares represented by those proxies as
to any other matters. Those individuals named in the Equity Office proxies
intend to vote or not vote in accordance
                                       89
<PAGE>   102

with the recommendation of the management of Equity Office. Those individuals
named as proxies in the Cornerstone proxies intend to vote or not vote in
accordance with the recommendation of the management of Cornerstone.

                      WHERE YOU CAN FIND MORE INFORMATION

     Equity Office has filed with the SEC a registration statement on Form S-4
(333-          ) of which this joint proxy statement/prospectus forms a part.
The registration statement registers the distribution to Cornerstone common
stockholders of the Equity Office common shares to be issued in the merger. The
registration statement, including the attached exhibits and schedules, contains
additional relevant information about Equity Office common shares. The rules and
regulations of the SEC allow us to omit certain information included in the
registration statement from this joint proxy statement/prospectus. In addition,
Equity Office and Cornerstone file reports, proxy statements and other
information with the SEC under the Securities Exchange Act of 1934. You may read
and copy any of this information at the following locations of the SEC:

Public Reference Room
450 Fifth Street, N.W.
Room 1024
Washington, D.C. 20549
New York Regional Office
7 World Trade Center
Suite 1300
New York, New York 10048
Chicago Regional Office
Citicorp Center
500 West Madison Street
Suite 1400
Chicago, Illinois 60661-2511

     You may obtain information on the operation of the SEC's Public Reference
Room by calling the SEC at 1-800-SEC-0330.

     The SEC also maintains an Internet web site that contains reports, proxy
statements and other information regarding issuers, including Equity Office and
Cornerstone, who file electronically with the SEC. The address of that site is
http://www.sec.gov. Reports, proxy statements and other information concerning
Equity Office and Cornerstone may also be inspected at the offices of the New
York Stock Exchange, which are located at 20 Broad Street, New York, New York
10005.

     The SEC allows Equity Office and Cornerstone to "incorporate by reference"
information in this document, which means that the companies can disclose
important information to you by referring you to another document filed
separately with the SEC. The information incorporated by reference is considered
to be a part of this joint proxy statement/prospectus, except for any
information that is superseded by information included directly in this
document.

     The documents listed below that Equity Office and Cornerstone have
previously filed with the SEC are considered to be a part of this joint proxy
statement/prospectus. They contain important information about our companies and
their financial condition.

     Equity Office SEC Filings (File No. 1-13115):

     Annual Report on Form 10-K Year ended December 31, 1999

    The description of Equity
    Office common shares set
    forth in the Equity Office
    registration statement filed
    under Section 12 of the
    Exchange Act on form 8-A
    on June 19, 1997

     Cornerstone SEC Filings (File No. 1-12861):

     Annual Report on Form 10-K Year ended December 31, 1999

                                       90
<PAGE>   103

     Equity Office and Cornerstone incorporate by reference additional documents
that either company may file with the SEC between the date of this joint proxy
statement/prospectus and the date of each company's special meeting. These
include periodic reports, such as Annual Reports on Form 10-K, Quarterly Reports
on Form 10-Q and Current Reports on Form 8-K, as well as proxy materials.

     If you are a shareholder of Equity Office or a stockholder of Cornerstone,
Equity Office or Cornerstone, as the case may be, may have sent to you some of
the documents incorporated by reference in this document, but you can obtain any
of them through Equity Office or Cornerstone, as the case may be, or through the
SEC or the SEC's web site described above. Documents incorporated by reference
are available from the companies, without charge, excluding all exhibits unless
specifically incorporated by reference as an exhibit to this document.
Shareholders of Equity Office or stockholders of Cornerstone may obtain
documents incorporated by reference in this document by requesting them from
writing or by telephone from the appropriate company at the following addresses:

    EQUITY OFFICE PROPERTIES TRUST
    Two North Riverside Plaza
    Suite 2100
    Chicago, Illinois 60606
    Attention: Stanley M. Stevens
    Telephone: (312) 466-3300

    CORNERSTONE PROPERTIES INC.
    Tower 56
    126 East 56th Street, 6th Floor
    New York, New York 10022
    Attention: Thomas P. Loftus
    Telephone: (212) 605-7131

     IF YOU WOULD LIKE TO REQUEST DOCUMENTS, PLEASE DO SO AT LEAST FIVE BUSINESS
DAYS BEFORE THE DATE OF THE SPECIAL MEETINGS IN ORDER TO RECEIVE TIMELY DELIVERY
OF SUCH DOCUMENTS BEFORE THE SPECIAL MEETINGS. If you request any incorporated
documents, we will mail them to you by first class mail, or another equally
prompt means, within one business day after we receive your request.

     Equity Office has supplied all information contained or incorporated by
reference in this joint proxy statement/prospectus relating to Equity Office, as
well as all pro forma financial information, and Cornerstone has supplied all
such information relating to Cornerstone. This document constitutes the
prospectus of Equity Office and a joint proxy statement of Cornerstone and
Equity Office.

                      WHAT INFORMATION YOU SHOULD RELY ON

     NO PERSON HAS BEEN AUTHORIZED TO GIVE ANY INFORMATION OR TO MAKE ANY
REPRESENTATION THAT DIFFERS FROM, OR ADDS TO, THE INFORMATION DISCUSSED IN THIS
JOINT PROXY STATEMENT/PROSPECTUS OR IN THE ANNEXES ATTACHED HERETO WHICH ARE
SPECIFICALLY INCORPORATED BY REFERENCE. THEREFORE, IF ANYONE GIVES YOU DIFFERENT
OR ADDITIONAL INFORMATION, YOU SHOULD NOT RELY ON IT.

     THIS DOCUMENT IS DATED             , 2000. THE INFORMATION CONTAINED IN
THIS JOINT PROXY STATEMENT/PROSPECTUS SPEAKS ONLY AS OF ITS DATE UNLESS THE
INFORMATION SPECIFICALLY INDICATES THAT ANOTHER DATE APPLIES. THIS JOINT PROXY
STATEMENT/PROSPECTUS DOES NOT CONSTITUTE AN OFFER TO EXCHANGE OR SELL, OR A
SOLICITATION OF AN OFFER TO EXCHANGE OR PURCHASE, EQUITY OFFICE COMMON SHARES OR
CORNERSTONE COMMON STOCK OR TO ASK FOR PROXIES, TO OR FROM ANY PERSON TO WHOM IT
IS UNLAWFUL TO DIRECT THESE ACTIVITIES.

                                       91
<PAGE>   104

                         EQUITY OFFICE PROPERTIES TRUST

               PRO FORMA CONDENSED COMBINED FINANCIAL STATEMENTS
                 AS OF AND FOR THE YEAR ENDED DECEMBER 31, 1999
                                  (UNAUDITED)

     The accompanying Unaudited Pro Forma Condensed Combined Balance Sheet as of
December 31, 1999 reflects the following transactions as if they had occurred on
December 31, 1999:

     - the proposed merger of Equity Office Properties Trust and Cornerstone
       Properties Inc.; and

     - the issuance of $500 million of unsecured notes on March 21, 2000, due
       March 2006, coupon rate of 8.4% per annum and effective rate of 8.5% per
       annum.

     The accompanying Unaudited Pro Forma Condensed Combined Statement of
Operations for the year ended December 31, 1999 reflects the following
transactions as if they had occurred on January 1, 1999:

     - the proposed merger of Equity Office Properties Trust and Cornerstone
       Properties Inc.; and

     - the issuance of $500 million of unsecured notes on March 21, 2000, due
       March 2006, coupon rate of 8.4% per annum and effective rate of 8.5% per
       annum.

     In the opinion of management, all significant adjustments necessary to
reflect the effects of the merger have been made.

                                       F-1
<PAGE>   105

                         EQUITY OFFICE PROPERTIES TRUST

                   PRO FORMA CONDENSED COMBINED BALANCE SHEET
                               DECEMBER 31, 1999
                                  (UNAUDITED)

<TABLE>
<CAPTION>
                                         EQUITY OFFICE       CORNERSTONE       MERGER       EQUITY OFFICE
                                        PROPERTIES TRUST   PROPERTIES INC.    AND OTHER    PROPERTIES TRUST
(DOLLARS IN THOUSANDS)                     HISTORICAL        HISTORICAL      ADJUSTMENTS      PRO FORMA
- ----------------------                  ----------------   ---------------   -----------   ----------------
                                                                                 (A)
<S>                                     <C>                <C>               <C>           <C>
ASSETS:
  Investment in real estate, net......    $12,572,153        $3,565,637      $  767,550(B)   $16,905,340
  Cash and cash equivalents...........          2,338            19,679               0(C)        22,017
  Rent and other receivables..........        193,194            91,968         (77,243)(D)       207,919
  Escrow deposits and restricted
     cash.............................         19,754           222,043               0          241,797
  Investment in unconsolidated joint
     ventures.........................        865,863            31,725           8,275(B)       905,863
  Prepaid expenses and other assets...        392,756           239,176        (199,275)(E)       432,657
                                          -----------        ----------      ----------      -----------
          Total Assets................    $14,046,058        $4,170,228      $  499,307      $18,715,593
                                          ===========        ==========      ==========      ===========
LIABILITIES AND SHAREHOLDERS' EQUITY:
  Mortgage debt.......................    $ 1,743,871        $1,448,331      $  (40,252)(F)   $ 3,151,950
  Unsecured notes.....................      3,655,047                --         499,320(G)     4,154,367
  Lines of credit.....................        453,000           329,000         684,827(H)     1,466,827
  Dividend/distribution payable.......          5,446                --               0            5,446
  Other liabilities...................        479,167           113,305               0          592,472
                                          -----------        ----------      ----------      -----------
          Total Liabilities...........      6,336,531         1,890,636       1,143,895        9,371,062
                                          -----------        ----------      ----------      -----------
  Commitments and contingencies
  Minority Interests:
     EOP Partnership..................        844,427           284,566          55,052(I)     1,184,045
     Partially owned properties.......         39,027            22,532               0           61,559
                                          -----------        ----------      ----------      -----------
          Total Minority Interests....        883,454           307,098          55,052        1,245,604
                                          -----------        ----------      ----------      -----------
  Shareholders' Equity:
     Preferred shares.................        615,000            50,000         (50,000)(J)       615,000
     Common shares....................          2,516                --             494(K)         3,010
     Additional paid in capital.......      6,208,557         1,922,494        (650,134)(L)     7,480,917
                                          -----------        ----------      ----------      -----------
          Total Shareholders'
            Equity....................      6,826,073         1,972,494        (699,640)       8,098,927
                                          -----------        ----------      ----------      -----------
          Total Liabilities and
            Shareholders' Equity......    $14,046,058        $4,170,228      $  499,307      $18,715,593
                                          ===========        ==========      ==========      ===========
</TABLE>

                                       F-2
<PAGE>   106

                         EQUITY OFFICE PROPERTIES TRUST
              PRO FORMA CONDENSED COMBINED STATEMENT OF OPERATIONS
                      FOR THE YEAR ENDED DECEMBER 31, 1999
                                  (UNAUDITED)

<TABLE>
<CAPTION>
                                                      EQUITY OFFICE       CORNERSTONE       MERGER       EQUITY OFFICE
                                                     PROPERTIES TRUST   PROPERTIES INC.    AND OTHER    PROPERTIES TRUST
(DOLLARS IN THOUSANDS, EXCEPT PER SHARE DATA)           HISTORICAL        HISTORICAL      ADJUSTMENTS      PRO FORMA
- ---------------------------------------------        ----------------   ---------------   -----------   ----------------
                                                                                              (A)
<S>                                                  <C>                <C>               <C>           <C>
Revenues:
  Rental...........................................    $ 1,493,196         $455,205        $  6,181(M)    $ 1,954,581
  Tenant reimbursements............................        281,358          121,385              --           402,743
  Parking..........................................        112,204           27,343              --           139,547
  Other............................................         32,298            5,730              --            38,028
  Fee income.......................................          8,939            2,677              --            11,616
  Interest/dividends...............................         14,248            4,211              --            18,459
                                                       -----------         --------        --------       -----------
        Total revenues.............................      1,942,243          616,550           6,181         2,564,974
                                                       -----------         --------        --------       -----------
Expenses:
  Interest:
    Expense incurred...............................        413,995          135,755          89,640(N)        639,390
    Amortization of deferred financing costs.......          4,693            2,004           3,669(O)         10,366
  Depreciation.....................................        339,751           87,971          16,310(P)        444,032
  Amortization.....................................         14,545            8,755          (8,755)(Q)        14,545
  Real estate taxes................................        243,778           71,554              --           315,332
  Insurance........................................          9,589            3,548              --            13,137
  Repairs and maintenance..........................        209,630           74,728              --           284,358
  Property operating...............................        199,879           52,947              --           252,826
  Ground rent......................................          6,887            1,407              --             8,294
  General and administrative.......................         80,927           27,006         (19,000)(R)        88,933
                                                       -----------         --------        --------       -----------
                                                         1,523,674          465,675          81,864         2,071,213
                                                       -----------         --------        --------       -----------
Income before allocation to minority interests,
  income from investment in unconsolidated joint
  ventures and net gain on sales of real estate....        418,569          150,875         (75,683)          493,761
Minority Interests:
  EOP Partnership..................................        (48,172)         (34,982)         (4,544)(S)       (87,698)
  Partially owned properties.......................         (1,981)          (5,755)             --            (7,736)
Income from investment in unconsolidated joint
  ventures.........................................         13,824              898            (176)(T)        14,546
Net gain on sales of real estate...................         59,661          131,034              --           190,695
                                                       -----------         --------        --------       -----------
Net income from continuing operations..............        441,901          242,070         (80,403)          603,568
Preferred distributions............................        (43,603)          (3,500)          3,500(U)        (43,603)
Put option settlement..............................         (5,658)              --              --            (5,658)
                                                       -----------         --------        --------       -----------
Net income from continuing operations before
  extraordinary items and cumulative effect of a
  change in accounting principle available for
  common shares....................................    $   392,640         $238,570        $(76,903)      $   554,307
                                                       ===========         ========        ========       ===========
Net income from continuing operations before
  extraordinary items and cumulative effect of a
  change in accounting principle per weighted
  average common share outstanding -- basic........           1.53(V)                                     $      1.81(V)
                                                       ===========                                        ===========
Weighted average common shares
  outstanding -- basic.............................    256,045,895                                        305,424,355
                                                       ===========                                        ===========
Net income from continuing operations before
  extraordinary items and cumulative effect of a
  change in accounting principle per weighted
  average common share outstanding -- diluted......    $      1.51(V)                                     $      1.81(V)
                                                       ===========                                        ===========
Weighted average common shares
  outstanding -- diluted...........................    291,157,204                                        353,945,132
                                                       ===========                                        ===========
</TABLE>

                                       F-3
<PAGE>   107

                         EQUITY OFFICE PROPERTIES TRUST

           NOTES TO PRO FORMA CONDENSED COMBINED FINANCIAL STATEMENTS
                               DECEMBER 31, 1999
                                  (UNAUDITED)
                 (DOLLARS IN THOUSANDS, EXCEPT PER SHARE DATA)

(A)  Represents adjustments to record the merger between Equity Office and
     Cornerstone based upon the assumed purchase price of $4.7 billion assuming
     a market value of $25.68125 per share of Equity Office's common shares and
     adjustments to record the issuance of $500 million in unsecured notes which
     occurred on March 21, 2000. The calculation of the purchase price of the
     merger is as follows:

<TABLE>
<S>                                                           <C>         <C>
Issuance of 49.4 million Equity Office common shares and
  13.4 million EOP Partnership units based on the .7009
  exchange rate, in exchange for 70.5 million shares of
  Cornerstone common stock and 19.1 million Cornerstone
  Partnership units.........................................              $1,612,472
Cash portion of merger consideration:
  Cornerstone common stock (58,551,525 @ $18.00)............  1,053,927
  Cornerstone preferred stock (3,030,303 @ $18.00)..........     54,545    1,108,472
                                                              ---------
Equity Office's current ownership of 250,700 shares of
  Cornerstone common stock..................................                   3,480
Assumption of Cornerstone's total liabilities...............               1,890,636
Adjustment to Cornerstone's mortgage debt to reflect market
  value (see Note F)........................................                 (40,252)
Cornerstone's minority interest allocation for partially
  owned properties..........................................                  22,532
Merger costs (see calculation below)........................                  71,800
                                                                          ----------
          Total.............................................              $4,669,140
                                                                          ==========
The following is a calculation of estimated merger costs:
Employee termination costs..................................              $   43,300
Investment advisory fees....................................                  15,000
Transfer taxes..............................................                   8,000
Legal, accounting and other fees............................                   4,000
Loan assumption fees........................................                   1,500
                                                                          ----------
          Total merger costs................................              $   71,800
                                                                          ==========
</TABLE>

(B)  Represents the estimated increase in Cornerstone's investment in real
     estate based upon Equity Office's purchase price to reflect the allocation
     to other tangible assets of Cornerstone being acquired:

<TABLE>
<S>                                                           <C>
Purchase price (see Note A).................................  $4,669,140
                                                              ----------
Less basis of Cornerstone's net assets acquired:
  Investment in real estate, net............................   3,565,637
  Cash and cash equivalents.................................      19,679
  Rent and other receivables (excluding $77.2 million of
     deferred rents receivable).............................      14,725
  Escrow deposits and restricted cash.......................     222,043
  Investment in unconsolidated joint ventures...............      31,725
  Prepaid expenses and other assets (excluding $199.7
     million of deferred leasing, financing and other costs
     (see Note E))..........................................      39,506
                                                              ----------
  Subtotal..................................................   3,893,315
                                                              ----------
</TABLE>

                                       F-4
<PAGE>   108
                         EQUITY OFFICE PROPERTIES TRUST

   NOTES TO PRO FORMA CONDENSED COMBINED FINANCIAL STATEMENTS -- (CONTINUED)
<TABLE>
<S>                                                           <C>
  Adjustment to record fair value of Cornerstone's
     investment in real estate and investment in
     unconsolidated joint ventures, net.....................     775,825
  Less adjustment allocated to investment in unconsolidated
     joint ventures.........................................      (8,275)
                                                              ----------
  Adjustment allocated to investment in real estate.........  $  767,550
                                                              ==========
</TABLE>

(C)  There was no change in cash as a result of the following transactions:

<TABLE>
<S>                                                           <C>
Anticipated borrowings on additional credit facilities to
  finance the cash portion of the merger (see Note H).......  $   684,827
Gross proceeds from the $500 million unsecured note offering
  (see Note G)..............................................      500,000
Less cash portion of merger consideration (see Note A)......   (1,108,472)
Less merger costs (see Note A)..............................      (71,800)
Less financing costs related to the $500 million unsecured
  note offering.............................................       (3,875)
Less discount on the $500 million unsecured note offering
  (see Note G)..............................................         (680)
                                                              -----------
  Net adjustment to cash and cash equivalents...............  $        --
                                                              ===========
</TABLE>

(D)  Represents the elimination of Cornerstone's deferred rents receivable of
     $77.2 million which arose from the historical straight-lining of rents.

(E)  Represents an adjustment for the following:

<TABLE>
<S>                                                           <C>
Cornerstone's deferred leasing, financing and other costs
  which were not assigned any value in the allocation of the
  purchase price of the merger..............................  $(199,670)
Equity Office's current ownership of 250,700 shares of
  Cornerstone common stock (see Note A).....................     (3,480)
Additional deferred financing costs associated with the $500
  million unsecured note offering (See Note G)..............      3,875
                                                              ---------
          Total.............................................  $(199,275)
                                                              =========
</TABLE>

(F)  To adjust Cornerstone's mortgage debt to market value.

(G)  To reflect the $500 million unsecured note offering net of a $.7 million
     discount.

(H)  To reflect borrowings on additional credit facilities to finance the cash
    portion of the merger as follows:

<TABLE>
<S>                                                           <C>
Cash portion of merger consideration (See Note A)...........  $1,108,472
Merger costs (See Note A)...................................      71,800
Less net proceeds from the $500 million unsecured notes
  offering (see Note C).....................................    (495,445)
                                                              ----------
Borrowings on additional credit facilities..................  $  684,827
                                                              ==========
</TABLE>

                                       F-5
<PAGE>   109
                         EQUITY OFFICE PROPERTIES TRUST

   NOTES TO PRO FORMA CONDENSED COMBINED FINANCIAL STATEMENTS -- (CONTINUED)

(I)  To adjust minority interests in the operating partnership to reflect the
     merger as follows:

<TABLE>
<S>                                                           <C>
Equity Office historical common shareholders' equity and
  minority interests in EOP Partnership.....................  $7,055,500
Cornerstone historical common shareholders' equity and
  minority interests in Cornerstone Partnership.............   2,207,060
Pro forma adjustments to common shareholders' equity........    (594,588)
                                                              ----------
          Total.............................................   8,667,972
Minority interests ownership percentage of EOP Partnership
  (see below)...............................................       13.66%
                                                              ----------
Minority interests in the equity of Equity Office after the
  merger....................................................   1,184,045
Less minority interests in the equity of Equity Office prior
  to the merger.............................................    (844,427)
Less minority interests in the equity of Cornerstone prior
  to the merger.............................................    (284,566)
                                                              ----------
Adjustment to minority interests in the equity of Equity
  Office to reflect the merger..............................  $   55,052
                                                              ==========
</TABLE>

     The 13.66% minority interests ownership in the equity of Equity Office is
     calculated as follows:

<TABLE>
<CAPTION>
                                                                            SHARES AND
                                                  SHARES         UNITS         UNITS
                                                -----------   -----------   -----------
<S>                                             <C>           <C>           <C>
Shares of Cornerstone common stock and
  Cornerstone Partnership units outstanding at
  December 31, 1999...........................  129,252,303    19,131,785   148,384,088
Less shares of Cornerstone common stock
  redeemed for cash (see Note A)..............  (58,551,525)           --   (58,551,525)
Less shares of Cornerstone common stock
  currently held by Equity Office (see Note
  A)..........................................     (250,700)           --      (250,700)
                                                -----------   -----------   -----------
Shares of Cornerstone common stock and
  Cornerstone Partnership units to be
  converted in the merger into Equity Office
  common shares and EOP Partnership units at
  the exchange rate of .7009..................   70,450,078    19,131,785    89,581,863
                                                ===========   ===========   ===========
Equity Office common shares and EOP
  Partnership units to be issued..............   49,378,460    13,409,468    62,787,928
Equity Office common shares and EOP
  Partnership units outstanding at December
  31, 1999....................................  251,582,434    34,203,912   285,786,346
                                                -----------   -----------   -----------
Equity Office common shares and EOP
  Partnership units outstanding after the
  merger......................................  300,960,894    47,613,380   348,574,274
                                                ===========   ===========   ===========
Minority interests ownership percentage of EOP
  Partnership.................................       13.66%
                                                ===========
Equity Office ownership percentage of EOP
  Partnership.................................       86.34%
                                                ===========
</TABLE>

(J)  To record the conversion in the merger of Cornerstone's preferred stock
     into cash (see Note A).

(K)  To record the par value of 49.4 million Equity Office common shares issued
     to Cornerstone common stockholders.

                                       F-6
<PAGE>   110
                         EQUITY OFFICE PROPERTIES TRUST

   NOTES TO PRO FORMA CONDENSED COMBINED FINANCIAL STATEMENTS -- (CONTINUED)

(L)  To reflect the net decrease in additional paid in capital associated with
     the merger as follows:

<TABLE>
<S>                                                           <C>
Issuance of 49.4 million EOP common shares and 13.4 million
  EOP Partnership units based on the .7009 exchange ratio,
  in exchange for 70.5 million shares of Cornerstone common
  stock and 19.1 million Cornerstone Partnership units (see
  Note A)...................................................  $ 1,612,472
Less par value of Equity Office common shares issued to
  Cornerstone common stockholders (see Note K)..............         (494)
Adjustment to minority interests in EOP Partnership (see
  Note I)...................................................      (55,052)
Less Cornerstone minority interest allocation to Cornerstone
  Partnership...............................................     (284,566)
Less Cornerstone additional paid in capital.................   (1,922,494)
                                                              -----------
     Net decrease in paid in capital........................  $  (650,134)
                                                              ===========
</TABLE>

(M) To reflect the adjustment for the straight-line effect of scheduled rent
    increases.

(N)  To reflect the additional interest expense incurred from the $500 million
     unsecured notes offering and borrowings from additional credit facilities
     to finance the cash portion of the merger.

<TABLE>
<S>                                                           <C>
Additional borrowings from $500 million unsecured notes (see
  Note G) and credit facilities (see Note H)................  $1,184,147
Weighted average interest rate..............................        7.57%
                                                              ----------
Additional interest expense.................................  $   89,640
                                                              ==========
</TABLE>

<TABLE>
<S>  <C>                                                           <C>
(O)  To reflect the amortization of $3.9 million of financing
     costs incurred with the $500 million unsecured notes over
     the six-year term...........................................   $  646
     To reflect the additional amortization of the mark-to-market
     adjustment of Cornerstone's fixed-rate debt.................    3,023
                                                                    ------
                                                                    $3,669
                                                                    ======
</TABLE>

(P)  To reflect the additional depreciation expense related to the adjustment to
     record the net equity value of the investment in real estate on a
     straight-line basis.

<TABLE>
<S>                                                           <C>
Adjustment to the basis of the investment in real estate
  (See Note B)..............................................  $767,550
Portion allocated to building and improvements, estimated to
  be 85%....................................................        85%
                                                              --------
Adjustment to the depreciable basis of Cornerstone's
  investment in real estate, net............................  $652,418
                                                              ========
Additional depreciation expense based on an estimated useful
  line of 40 years..........................................  $ 16,310
                                                              ========
</TABLE>

(Q)  To reverse Cornerstone's historical deferred lease amortization expense due
     to the write-off of deferred lease costs as a result of the merger (See
     Note E).

(R)  To reflect the anticipated reduction of general and administrative expenses
     as a result of the merger (See Note E).

(S)  To reflect the estimated minority interests ownership in EOP Partnership
     after the merger of 13.66%.

                                       F-7
<PAGE>   111
                         EQUITY OFFICE PROPERTIES TRUST

   NOTES TO PRO FORMA CONDENSED COMBINED FINANCIAL STATEMENTS -- (CONTINUED)

(T)  To reflect the additional depreciation expense on the adjustment to the
     depreciable basis of Cornerstone's investment in unconsolidated joint
     ventures.

<TABLE>
<S>                                                           <C>
Adjustment to the basis of the investment in unconsolidated
  joint ventures............................................   $8,275
Portion allocated to building and improvements, estimated to
  be 85%....................................................       85%
                                                               ------
Adjusted to the depreciable basis of Cornerstone's
  investment in unconsolidated joint ventures...............   $7,034
                                                               ======
Decrease in income from investment in unconsolidated joint
  ventures due to additional depreciation expense pertaining
  to the adjustment to the basis of the investment in joint
  ventures based on an estimated useful life of 40 years....   $  176
                                                               ======
</TABLE>

(U)  To adjust Cornerstone's preferred distributions assuming the shares of
     Cornerstone preferred stock were converted into cash on January 1, 1999.

(V)  The following tables set forth the computation of basic and diluted
     earnings per common share:

<TABLE>
<CAPTION>
NUMERATOR                                                      HISTORICAL     PRO FORMA
- ---------                                                     ------------   ------------
<S>                                                           <C>            <C>
Net income from continuing operations before extraordinary
  items, cumulative effect of a change in accounting
  principle and net gain on sales of real estate available
  for common shares.........................................  $    332,979   $    363,612
Net gain on sales of real estate............................        59,661        190,695
                                                              ------------   ------------
Numerator for basic earnings per share -- net income from
  continuing operations before extraordinary items and
  cumulative effect of a change in accounting principle
  available for common shares...............................       392,640        554,307
Minority interest in EOP Partnership........................        48,172         87,698
                                                              ------------   ------------
Numerator for diluted earnings per share -- net income from
  continuing operations before extraordinary items and
  cumulative effect of a change in accounting principle
  available for common shares...............................  $    440,812   $    642,005
                                                              ------------   ------------
DENOMINATOR
- ------------------------------------------------------------
Denominator for basic earnings per share -- weighted average
  common shares.............................................   256,045,895    305,424,355
                                                              ------------   ------------
Denominator for diluted earnings per share -- weighted
  average common shares.....................................   291,157,204    353,945,132
                                                              ------------   ------------
BASIC
- ------------------------------------------------------------
Net income from continuing operations before extraordinary
  items, cumulative effect of a change in accounting
  principle and net gain on sales of real estate available
  for common shares.........................................  $       1.30   $       1.19
Net gain on sales of real estate............................          0.23           0.62
                                                              ------------   ------------
Net income from continuing operations before extraordinary
  items, cumulative effect of a change in accounting
  principle available for common shares.....................  $       1.53   $       1.81
                                                              ============   ============
</TABLE>

                                       F-8
<PAGE>   112
                         EQUITY OFFICE PROPERTIES TRUST

   NOTES TO PRO FORMA CONDENSED COMBINED FINANCIAL STATEMENTS -- (CONTINUED)

<TABLE>
<CAPTION>
DILUTED                                                        HISTORICAL     PRO FORMA
- -------                                                       ------------   ------------
<S>                                                           <C>            <C>
Net income from continuing operations before extraordinary
  items cumulative effect of a change in accounting
  principle and net gain on sales of real estate available
  for common shares.........................................  $       1.31   $       1.28
Net gain on sales of real estate............................          0.20           0.53
                                                              ------------   ------------
Net income from continuing operations before extraordinary
  items, cumulative effect of a change in accounting
  principle available for common shares.....................  $       1.51   $       1.81
                                                              ============   ============
</TABLE>

                                       F-9
<PAGE>   113

                                                                         ANNEX A

                          AGREEMENT AND PLAN OF MERGER
                                     AMONG
                        EQUITY OFFICE PROPERTIES TRUST,
                       EOP OPERATING LIMITED PARTNERSHIP,
                          CORNERSTONE PROPERTIES INC.
                                      AND
                   CORNERSTONE PROPERTIES LIMITED PARTNERSHIP
                         DATED AS OF FEBRUARY 11, 2000
<PAGE>   114

                               TABLE OF CONTENTS

<TABLE>
<CAPTION>
                                                                           PAGE
                                                                           ----
<C>          <S>                                                           <C>
ARTICLE 1    THE MERGERS.................................................   A-2
    1.1      Election by Limited Partners in Cornerstone Partnership to
             Exercise the Redemption Right; The Partnership Merger.......   A-2
    1.2      The Merger..................................................   A-3
    1.3      Closing.....................................................   A-3
    1.4      Effective Time..............................................   A-3
    1.5      Effect of Partnership Mergers on Agreement of Limited
             Partnership.................................................   A-4
    1.6      Effect of Merger on Declaration of Trust and Bylaws.........   A-4
    1.7      Trustees of EOP.............................................   A-4
    1.8      Effect on Shares............................................   A-5
    1.9      Effect on Partnership Interests.............................   A-5
   1.10      Exchange Ratios and Other Merger Consideration..............   A-5
   1.11      Election by Holders of Cornerstone Common Stock to Receive
             EOP Common Shares or Cash...................................   A-6
   1.12      Proration...................................................   A-7
   1.13      Partner Approval............................................   A-8
   1.14      No Appraisal Rights.........................................   A-8
   1.15      Exchange of Certificates; Pre-Closing Dividends; Fractional
             Shares......................................................   A-8
ARTICLE 2    REPRESENTATIONS AND WARRANTIES OF CORNERSTONE AND
             CORNERSTONE PARTNERSHIP.....................................  A-11
    2.1      Organization, Standing and Power............................  A-12
    2.2      Cornerstone Subsidiaries....................................  A-12
    2.3      Capital Structure...........................................  A-13
    2.4      Other Interests.............................................  A-14
    2.5      Authority; Noncontravention; Consents.......................  A-14
    2.6      SEC Documents; Financial Statements; Undisclosed
             Liabilities.................................................  A-16
    2.7      Absence of Certain Changes or Events........................  A-16
    2.8      Litigation..................................................  A-17
    2.9      Properties..................................................  A-17
   2.10      Environmental Matters.......................................  A-19
   2.11      Related Party Transactions..................................  A-20
   2.12      Employee Benefits...........................................  A-20
   2.13      Employee Policies...........................................  A-21
   2.14      Taxes.......................................................  A-22
   2.15      No Payments to Employees, Officers or Directors.............  A-23
   2.16      Broker; Schedule of Fees and Expenses.......................  A-23
   2.17      Compliance with Laws........................................  A-23
   2.18      Contracts; Debt Instruments.................................  A-23
   2.19      Opinion of Financial Advisor................................  A-25
   2.20      State Takeover Statutes.....................................  A-25
   2.21      Investment Company Act of 1940..............................  A-25
   2.22      Definition of Knowledge of Cornerstone......................  A-25
   2.23      Required Stockholder Approvals and Partner Approvals........  A-25
ARTICLE 3    REPRESENTATIONS AND WARRANTIES OF EOP AND EOP PARTNERSHIP...  A-26
    3.1      Organization, Standing and Power of EOP.....................  A-26
    3.2      EOP Subsidiaries............................................  A-26
    3.3      Capital Structure...........................................  A-27
    3.4      Other Interests.............................................  A-28
</TABLE>

                                        i
<PAGE>   115

<TABLE>
<CAPTION>
                                                                           PAGE
                                                                           ----
<C>          <S>                                                           <C>
    3.5      Authority; Noncontravention; Consents.......................  A-28
    3.6      SEC Documents; Financial Statements; Undisclosed
             Liabilities.................................................  A-29
    3.7      Absence of Certain Changes or Events........................  A-29
    3.8      Litigation..................................................  A-30
    3.9      Properties..................................................  A-30
   3.10      Environmental Matters.......................................  A-31
   3.11      Taxes.......................................................  A-32
   3.12      Brokers; Schedule of Fees and Expenses......................  A-33
   3.13      Compliance with Laws........................................  A-33
   3.14      Contracts; Debt Instruments.................................  A-33
   3.15      Opinion of Financial Advisor................................  A-33
   3.16      State Takeover Statutes.....................................  A-33
   3.17      Investment Company Act of 1940..............................  A-33
   3.18      Definition of Knowledge of EOP..............................  A-33
   3.19      Required Shareholder Approvals and Partner Approvals........  A-33
ARTICLE 4    COVENANTS...................................................  A-34
    4.1      Conduct of Cornerstone's and Cornerstone Partnership's
             Business Pending Merger.....................................  A-34
    4.2      Conduct of EOP's and EOP Partnership's Business Pending
             Merger......................................................  A-36
    4.3      No Solicitation.............................................  A-38
    4.4      Affiliates..................................................  A-39
    4.5      Other Actions...............................................  A-40
ARTICLE 5    ADDITIONAL COVENANTS........................................  A-40
    5.1      Preparation of the Form S-4 and the Proxy Statement;
             Cornerstone Stockholders Meeting, Cornerstone Unitholders
             Consent Solicitation and EOP Shareholders Meeting...........  A-40
    5.2      Access to Information; Confidentiality......................  A-42
    5.3      Commercially Reasonable Efforts; Notification...............  A-42
    5.4      Tax Matters.................................................  A-43
    5.5      Public Announcements........................................  A-44
    5.6      Listing.....................................................  A-45
    5.7      Transfer and Gains Taxes....................................  A-45
    5.8      Benefit Plans and Other Employee Arrangements...............  A-45
    5.9      Indemnification.............................................  A-46
   5.10      Declaration of Dividends and Distributions..................  A-48
   5.11      Transfer of Non-Controlled Subsidiary Voting Shares.........  A-49
   5.12      Notices.....................................................  A-49
   5.13      Resignations................................................  A-49
   5.14      Assumption of Existing Tax Protection Agreements............  A-49
   5.15      EOP Partnership Agreement...................................  A-49
   5.16      Registration Rights Agreements..............................  A-49
   5.17      Cornerstone Convertible Promissory Note.....................  A-49
ARTICLE 6    CONDITIONS..................................................  A-50
    6.1      Conditions to Each Party's Obligation to Effect the
             Mergers.....................................................  A-50
    6.2      Conditions to Obligations of EOP and EOP Partnership........  A-50
    6.3      Conditions to Obligations of Cornerstone and Cornerstone
             Partnership.................................................  A-51
ARTICLE 7    TERMINATION, AMENDMENT AND WAIVER...........................  A-53
    7.1      Termination.................................................  A-53
    7.2      Certain Fees and Expenses...................................  A-54
    7.3      Effect of Termination.......................................  A-55
    7.4      Amendment...................................................  A-55
</TABLE>

                                       ii
<PAGE>   116

<TABLE>
<CAPTION>
                                                                           PAGE
                                                                           ----
<C>          <S>                                                           <C>
    7.5      Extension; Waiver...........................................  A-55
ARTICLE 8    GENERAL PROVISIONS..........................................  A-56
    8.1      Nonsurvival of Representations and Warranties...............  A-56
    8.2      Notices.....................................................  A-56
    8.3      Interpretation..............................................  A-57
    8.4      Counterparts................................................  A-57
    8.5      Entire Agreement; No Third-Party Beneficiaries..............  A-57
    8.6      Governing Law...............................................  A-57
    8.7      Assignment..................................................  A-57
    8.8      Enforcement.................................................  A-57
    8.9      Severability................................................  A-57
   8.10      Exculpation.................................................  A-58
   8.11      Joint and Several Obligations...............................  A-58
</TABLE>

                                    EXHIBITS

Exhibit "A" -- Form of Certificate of Merger

Exhibit "B" -- Form of Maryland Articles of Merger

Exhibit "C" -- Form of Nevada Articles of Merger

Exhibit "D" -- Form of Proposed EOP Charter Amendment Relating to Certain Voting
               Requirements

Exhibit "E" -- Form of Proposed EOP Charter Amendment Relating to Domestically
               Controlled REIT Status

                                       iii
<PAGE>   117

                             INDEX OF DEFINED TERMS

<TABLE>
<S>                                                           <C>
AICPA Statement.............................................  5.1(b)
Acquisition Proposal........................................  4.3(a)(i)
Affiliate...................................................  2.11
Agreement...................................................  Preamble
Base Amount.................................................  7.2
BeaMet......................................................  3.11(b)
Break-Up Fee................................................  7.2
Break-Up Fee Tax Opinion....................................  7.2
Break-Up Expenses...........................................  7.2
Cash Election...............................................  1.11(a)
Cash Election Shares........................................  1.12(a)
Cash Fraction...............................................  1.12(b)
CERCLA......................................................  2.10(a)
Certificate of Merger.......................................  C
Certificate.................................................  1.10(b)(iv)
Closing.....................................................  1.3
Closing Date................................................  1.3
Cornerstone.................................................  Preamble
Cornerstone Acquisition Agreement...........................  7.2
Cornerstone Articles........................................  2.1
Cornerstone Bylaws..........................................  2.1
Cornerstone Common Stock....................................  1.10(b)(i)
Cornerstone Convertible Promissory Note.....................  2.3(c)
Cornerstone Disclosure Letter...............................  Art. 2
Cornerstone Financial Statement Date........................  2.7
Cornerstone Material Adverse Change.........................  2.7
Cornerstone Material Adverse Effect.........................  2.1
Cornerstone Non-controlled Subsidiary.......................  J
Cornerstone OP Unit.........................................  1.1(a)
Cornerstone Other Interests.................................  2.4
Cornerstone Partner Approvals...............................  1.13
Cornerstone Partnership.....................................  Preamble
Cornerstone Partnership Agreement...........................  1.5
Cornerstone Preferred OP Unit...............................  1.10(a)(ii)
Cornerstone Properties......................................  2.9(a)
Cornerstone Rent Roll.......................................  2.9(e)
Cornerstone SEC Documents...................................  2.6
Cornerstone 7% Preferred Stock..............................  1.10(b)(iii)
Cornerstone Stockholder Approvals...........................  2.5(a)
Cornerstone Stockholders Meeting............................  5.1(d)
Cornerstone Space Lease.....................................  2.9(e)
Cornerstone Stock Options...................................  2.3(b)
Cornerstone Stock Rights....................................  2.3(b)
Cornerstone Subsidiaries....................................  2.2(a)
Cornerstone Voting Agreement................................  K
Code........................................................  F
Commitment..................................................  4.1(i)
Common Stock Exchange Ratio.................................  1.10(b)(ii)
Confidentiality Agreement...................................  2.18(k)
Controlled Group Member.....................................  2.12
Department..................................................  1.4
</TABLE>

                                       iv
<PAGE>   118
<TABLE>
<S>                                                           <C>
DRULPA......................................................  1.1(b)
Effective Time..............................................  1.4
Electing Cornerstone OP Units...............................  1.11
Election....................................................  1.11(a)
Election Date...............................................  1.11(d)
Employee Plan...............................................  2.12
Encumbrances................................................  2.9(a)
Environmental Law...........................................  2.10(a)
Environmental Permits.......................................  2.10(b)(iv)
EOP.........................................................  Preamble
EOP Bylaws..................................................  1.6
EOP Common Share............................................  1.10(b)(ii)
EOP Counter Proposal........................................  4.3(c)
EOP Declaration of Trust....................................  1.6
EOP Disclosure Letter.......................................  Art. 3
EOP Financial Statement Date................................  3.7
EOP Material Adverse Change.................................  3.7
EOP Material Adverse Effect.................................  3.1
EOP NCS Sub.................................................  J
EOP Options.................................................  3.3(b)
EOP OP Unit.................................................  1.10(a)(i)
EOP Other Interests.........................................  3.4
EOP Partner Approvals.......................................  1.13
EOP Partnership.............................................  Preamble
EOP Partnership Agreement...................................  1.5
EOP Preferred OP Unit.......................................  1.10(a)(ii)
EOP Preferred Units.........................................  3.3(e)
EOP Preferred Shares........................................  3.3(a)
EOP Properties..............................................  3.9(a)
EOP Rent Roll...............................................  3.9(g)
EOP SEC Documents...........................................  3.6
EOP Shareholder Approvals...................................  3.5(a)
EOP Shareholders Meeting....................................  5.1(c)
EOP Space Lease.............................................  3.9(g)
EOP Subsidiaries............................................  3.1
ERISA.......................................................  2.12
Exchange Act................................................  2.6
Exchange Agent..............................................  1.15(a)
Exchange Fund...............................................  1.15(b)
Exercise....................................................  1.1(a)(i)
Final Company Dividend......................................  1.15(d)(i)
Form of Election............................................  1.11(b)
Form S-4....................................................  5.1(a)
Former Cornerstone Properties...............................  2.10(b)(ii)
GAAP........................................................  2.6
Governmental Entity.........................................  2.5(c)
Hazardous Materials.........................................  2.10
HSR Act.....................................................  2.5(c)
Indebtedness................................................  2.18(b)
Indemnification Parties.....................................  5.9(b)
Indemnified Parties.........................................  5.9(a)
Indemnifying Parties........................................  5.9(a)
Joint Proxy Statement.......................................  5.1(a)
</TABLE>

                                        v
<PAGE>   119
<TABLE>
<S>                                                           <C>
Knowledge of Cornerstone....................................  2.22
Knowledge of EOP............................................  3.18
Laws........................................................  2.5(c)
Liens.......................................................  2.2(b)
Maryland Articles of Merger.................................  D
Maximum Amount..............................................  7.2
Merger......................................................  A
Mergers.....................................................  B
Merger Consideration........................................  1.10(b)
Nevada Articles of Merger...................................  E
Non-Electing Shares.........................................  1.12(e)
NRS.........................................................  1.2
NYSE........................................................  1.15(g)(ii)
Partner Approvals...........................................  1.13
Partnership Merger..........................................  B
Payor.......................................................  7.2
Pension Plan................................................  2.12
Permitted Title Exceptions..................................  2.9(a)
Person......................................................  2.2(a)
PGGM........................................................  6.3(h)
Prohibited Transaction......................................  2.12(c)
Property Restrictions.......................................  2.9(a)
Proposed EOP Charter Amendment Relating to Domestically
  Controlled REIT Status....................................  4.2(h)
Proposed EOP Charter Amendment Relating to Certain Voting
  Requirements..............................................  4.2(h)
Proposed EOP Charter Amendments.............................  4.2(h)
Qualifying Income...........................................  7.2
Recipient...................................................  7.2
REIT........................................................  2.14(b)
REIT Requirements...........................................  7.2
Release.....................................................  2.10(a)
Rule 145 Affiliates.........................................  4.4
SEC.........................................................  2.5(c)
Section 704(c) values.......................................  5.4(b)
Securities Act..............................................  2.3(g)
Share Election..............................................  1.11(a)
Shareholder Approvals.......................................  3.5(a)
Stock Election Shares.......................................  1.12(a)
Stock Fraction..............................................  1.12(c)
Stock Purchase Agreement....................................  J
Subsidiary..................................................  2.2(a)
Substituted Option..........................................  5.8(c)
Superior Acquisition Proposal...............................  4.3(d)
Surviving Trust.............................................  1.2
Takeover Statute............................................  2.20
Taxes.......................................................  2.14(a)
Tax Protection Agreements...................................  2.18(j)
Title 3.....................................................  1.2
Title 8.....................................................  1.2
Transfer and Gains Taxes....................................  5.7
Welfare Plan................................................  2.12
1940 Act....................................................  2.21
</TABLE>

                                       vi
<PAGE>   120

                          AGREEMENT AND PLAN OF MERGER

     THIS AGREEMENT AND PLAN OF MERGER (this "Agreement"), dated as of February
11, 2000, by and among EQUITY OFFICE PROPERTIES TRUST, a Maryland real estate
investment trust ("EOP"), EOP OPERATING LIMITED PARTNERSHIP, a Delaware limited
partnership ("EOP Partnership"), CORNERSTONE PROPERTIES INC., a Nevada
corporation ("Cornerstone"), and CORNERSTONE PROPERTIES LIMITED PARTNERSHIP, a
Delaware limited partnership ("Cornerstone Partnership").

                                   RECITALS:

     A. The Board of Trustees of EOP and the Board of Directors of Cornerstone
deem it advisable and in the best interests of their respective shareholders and
stockholders, upon the terms and subject to the conditions contained herein,
that Cornerstone shall merge with and into EOP (the "Merger").

     B. EOP, as the managing general partner of EOP Partnership, and
Cornerstone, as the sole general partner of Cornerstone Partnership, deem it
advisable and in the best interests of their respective limited partners,
subject to the conditions and other provisions contained herein, that,
immediately prior to the Merger, Cornerstone Partnership shall merge with and
into EOP Partnership, with the holders of partnership interests in Cornerstone
Partnership at the time of the Partnership Merger receiving in any event units
of limited partnership interest in EOP Partnership, as set forth herein (the
"Partnership Merger" and, together with the Merger, the "Mergers"). As an
alternative to receiving units of limited partnership interest in EOP
Partnership in connection with the Partnership Merger, limited partners in
Cornerstone Partnership (other than Cornerstone) shall have the right to elect,
effective immediately prior to the Partnership Merger, to exercise their
redemption right under the Cornerstone Partnership Agreement (as defined
herein), regardless of whether or not they would otherwise be entitled to
exercise that redemption right under the Cornerstone Partnership Agreement, and
Cornerstone shall issue shares of Cornerstone Common Stock (as defined herein)
in satisfaction of that right, thereby allowing former limited partners in
Cornerstone Partnership (other than Cornerstone) to participate in the Merger as
holders of Cornerstone Common Stock.

     C. Upon the terms and subject to the conditions set forth herein,
immediately prior to the Merger, EOP Partnership and Cornerstone Partnership
shall execute a Certificate of Merger (the "Certificate of Merger") in
substantially the form attached hereto as Exhibit A and shall file such
Certificate of Merger in accordance with Delaware law to effectuate the
Partnership Merger.

     D. Upon the terms and subject to the conditions set forth herein,
immediately following the effectiveness of the Partnership Merger, EOP and
Cornerstone shall execute Articles of Merger (the "Maryland Articles of Merger")
in substantially the form attached hereto as Exhibit B and shall file such
Maryland Articles of Merger in accordance with Maryland law to effectuate the
Merger.

     E. Upon the terms and subject to the conditions set forth herein,
immediately following the effectiveness of the Partnership Merger, EOP and
Cornerstone shall execute Articles of Merger (the "Nevada Articles of Merger")
in substantially the form attached hereto as Exhibit C and concurrently with the
filing of the Maryland Articles of Merger, shall file such Nevada Articles of
Merger in accordance with Nevada law to effectuate the Merger.

     F. For federal income tax purposes, it is intended that the Merger shall
qualify as a reorganization under Section 368(a) of the Internal Revenue Code of
1986, as amended (the "Code"), and that this Agreement shall constitute a plan
of reorganization under Section 368(a) of the Code.

     G. For federal income tax purposes, it is intended that the Partnership
Merger, regardless of form, be treated as a contribution by Cornerstone
Partnership of all of its assets to EOP Partnership in exchange for partnership
interests in EOP Partnership, as provided for herein, under Section 721 of the
Code, and a distribution of such partnership interests by Cornerstone
Partnership to its partners under Section 731 of the Code.
                                       A-1
<PAGE>   121

     H. EOP and Cornerstone have each received a fairness opinion relating to
the transactions contemplated hereby as more fully described herein.

     I. EOP, EOP Partnership, Cornerstone and Cornerstone Partnership desire to
make certain representations, warranties and agreements in connection with the
Mergers.

     J. Concurrently with the execution of this Agreement and as an inducement
to EOP and EOP Partnership to enter into this Agreement, William Wilson III and
John S. Moody, as the owners of 99% of the voting capital stock of WCP Services,
Inc., a Delaware corporation (the "Cornerstone Non-controlled Subsidiary"), have
entered into a Stock Purchase Agreement, dated as of the date hereof, relating
to the voting capital stock of the Cornerstone Non-controlled Subsidiary (the
"Stock Purchase Agreement"), providing for the sale of all of the outstanding
voting capital stock of the Cornerstone Non-controlled Subsidiary to EOP Office
Properties Management Corporation ("EOP NCS Sub") or its assigns.

     K. As an inducement to EOP to enter into this Agreement, (a) Stichting
Pensioenfonds voor de Gezondheid, Geestelijke en Maatschappelijke Belangen, a
stichting formed according to the laws of the Kingdom of The Netherlands, each
of the directors and certain executive officers of Cornerstone (and the spouses
of certain of the foregoing) and certain entities controlled by any of the
foregoing have entered into a voting agreement (each, a "Cornerstone Voting
Agreement"), pursuant to which such person or entity has agreed, among other
things, to vote his or its shares of Cornerstone Common Stock and Cornerstone OP
Units (as defined herein) to approve this Agreement, the respective Mergers and
any other matter which requires his or its vote in connection with the
transactions contemplated by this Agreement and (b) the holders of the
outstanding Cornerstone 7% Preferred Stock (as defined herein) have entered into
a stock option agreement, pursuant to which such holders have granted
Cornerstone and EOP an option to acquire, among other things, all of their
Cornerstone 7% Preferred Stock at any time prior to the Effective Time (as
defined herein) of the Merger at a per share purchase price of $18.00, together
with accrued and unpaid dividends to the Effective Time, in cash (subject to
adjustment).

     L. As an inducement to Cornerstone to enter into this Agreement, each of
the trustees and certain executive officers of EOP (and the spouses of certain
of the foregoing) and certain entities controlled by any of the foregoing have
entered into a voting agreement pursuant to which such person or entity has
agreed, among other things, to vote his or its EOP Common Shares (as defined
herein) to approve this Agreement, the Merger and any other matter which
requires his or its vote in connection with the transactions contemplated by
this Agreement.

     NOW, THEREFORE, in consideration of the premises and the mutual
representations, warranties, covenants and agreements contained herein, the
parties hereto hereby agree as follows:

                                   ARTICLE 1

                                  THE MERGERS

     1.1 Election by Limited Partners in Cornerstone Partnership to Exercise the
Redemption Right; The Partnership Merger.

     (a) Notwithstanding any limitation or restriction contained in the
Cornerstone Partnership Agreement with respect to the ability of a Limited
Partner (as defined in the Cornerstone Partnership Agreement) to exercise the
Redemption Right (as defined in the Cornerstone Partnership Agreement)
(including, without limitation, any limitation or restriction contained in
Section 8.6A of the Cornerstone Partnership Agreement), every Limited Partner
shall have the right to exercise the Redemption Right by submitting to
Cornerstone Partnership (with a copy to Cornerstone) during the period between
the mailing date of the Joint Proxy Statement (as defined herein) for the
Cornerstone Stockholders Meeting (as defined herein) and 5:00 p.m., Eastern
time, on the second business day prior to the date of the Cornerstone
Stockholders Meeting a Notice of Redemption (as defined in the Cornerstone
Partnership Agreement) specifying the number of Class A Partnership Common Units
(as defined in the Cornerstone Partnership Agreement) of Cornerstone Partnership
(the "Cornerstone OP Units") which such Limited Partner desires to have

                                       A-2
<PAGE>   122

redeemed pursuant to Section 8.6A of the Cornerstone Partnership Agreement (as
modified by this Section 1.1(a)), which Notice of Redemption shall be
conditioned upon the closing of the Partnership Merger and can be conditional as
set forth in clause (v) below; provided, that,

          (i) with respect to each Notice of Redemption (a copy of the form of
     which shall accompany or form a part of the Form of Election (as defined
     herein)) properly submitted by a Limited Partner in accordance with this
     Section 1.1(a) (an "Exercise"), Cornerstone shall elect in accordance with
     Section 8.6B of the Cornerstone Partnership Agreement to purchase the
     Cornerstone OP Units relating to such Exercise by paying the REIT Shares
     Amount (as defined in the Cornerstone Partnership Agreement) and not the
     Cash Amount (as defined in the Cornerstone Partnership Agreement);

          (ii) notwithstanding the provisions of Section 8.6B of the Cornerstone
     Partnership Agreement, Cornerstone shall not be required to notify the
     Redeeming Partner (as defined in the Cornerstone Partnership Agreement) of
     Cornerstone's election to purchase the Cornerstone OP Units as described in
     the foregoing clause (i);

          (iii) the Specified Redemption Date (as defined in the Cornerstone
     Partnership Agreement) shall be the Closing Date (as defined herein) at a
     time prior to the consummation of the Partnership Merger;

          (iv) each Redeeming Partner shall be treated as an owner of the shares
     of Cornerstone Common Stock issued pursuant to this Agreement at the
     Effective Time (as defined herein) of the Merger and shall have the same
     right as each of the other holders of shares of Cornerstone Common Stock to
     make an Election (as defined herein) pursuant to Section 1.11 as to the
     form of consideration to be received in the Merger with respect to such
     shares of Cornerstone Common Stock; and

          (v) a Redeeming Partner shall have the option, in its discretion, to
     make its Notice of Redemption conditional upon part or all of the shares of
     Cornerstone Common Stock that would be issued pursuant thereto being
     converted solely into the right to receive cash in the Merger pursuant to
     Section 1.10(b)(i) and the procedures set forth in Sections 1.11 and 1.12,
     in which event the Notice of Redemption shall not be effective with respect
     to any Cornerstone OP Units for which the shares of Cornerstone Common
     Stock that would be received therefor would not be converted entirely into
     the right to receive cash in the Merger (and any Cornerstone OP Units not
     redeemed as a result thereof would be converted into EOP OP Units (as
     defined herein) in the Partnership Merger as set forth in Section
     1.10(a)(i)).

     (b) Upon the terms and subject to the conditions of this Agreement, and in
accordance with Title 6, Chapter 17 of the Delaware Code Annotated, as amended
(the "DRULPA"), immediately prior to the consummation of the Merger, Cornerstone
Partnership shall be merged with and into EOP Partnership, with EOP Partnership
as the surviving limited partnership or limited liability company, and with the
holders of partnership interests in Cornerstone Partnership receiving in any
event units of partnership interest in EOP Partnership, as set forth in Section
1.10.

     1.2 The Merger. Upon the terms and subject to the conditions set forth in
this Agreement, and in accordance with Title 3 of the Corporations and
Associations Article of the Annotated Code of Maryland, as amended ("Title 3"),
Title 8 of the Corporations and Associations Article of the Annotated Code of
Maryland, as amended ("Title 8"), and Chapter 92A of Title 7 of the Nevada
Revised Statutes Annotated (the "NRS"), immediately following the effectiveness
of the Partnership Merger, Cornerstone shall be merged with and into EOP, with
EOP surviving as a real estate investment trust (the "Surviving Trust").

     1.3 Closing. The closing of the Mergers (the "Closing") will take place
commencing at 9:00 a.m., local time, on the date to be specified by the parties,
which (subject to satisfaction or waiver of the conditions set forth in Article
6) shall be no later than the third business day after satisfaction or waiver of
the conditions set forth in Section 6.1(a) (the "Closing Date"), at the offices
of Hogan & Hartson L.L.P., 555 Thirteenth Street, N.W., Washington, D.C. 20004,
unless another date or place is agreed to in writing by the parties.
                                       A-3
<PAGE>   123

     1.4 Effective Time. As soon as practicable following the satisfaction or
waiver of the conditions set forth in Article 6, (i) EOP Partnership and
Cornerstone Partnership shall execute and file the Certificate of Merger,
executed in accordance with the DRULPA, with the Office of the Secretary of
State of the State of Delaware, and (ii) EOP and Cornerstone shall then execute
and file the Maryland Articles of Merger, executed in accordance with Title 3
and Title 8, with the State Department of Assessments and Taxation of Maryland
(the "Department"), and the Nevada Articles of Merger, executed in accordance
with Title 7 of the NRS with the Secretary of State of the State of Nevada, and
shall make all other filings and recordings required, with respect to the
Partnership Merger, under the DRULPA or, with respect to the Merger, under Title
3, Title 8 and the NRS. The Mergers shall become effective (each an "Effective
Time" and collectively the "Effective Times") at such times as EOP and
Cornerstone shall agree should be specified in the Certificate of Merger, the
Maryland Articles of Merger and the Nevada Articles of Merger (not to exceed
thirty (30) days after the Maryland Articles of Merger are accepted for record
by the Department). Unless otherwise agreed, the parties shall cause the
Effective Times to occur on the Closing Date, with not less than one hour
between the Effective Time of the Partnership Merger and the Effective Time of
the Merger.

     1.5 Effect of Partnership Merger on Agreements of Limited Partnership. The
Agreement of Limited Partnership, as amended, of EOP Partnership, as in effect
immediately prior to the Effective Time of the Partnership Merger (the "EOP
Partnership Agreement"), shall continue in full force and effect after the
Partnership Merger until further amended in accordance with applicable Delaware
law. The Agreement of Limited Partnership, as amended, of Cornerstone
Partnership, as in effect immediately prior to the Effective Time of the
Partnership Merger (the "Cornerstone Partnership Agreement") shall terminate at
the Effective Time of Partnership Merger.

     1.6 Effect of Merger on Declaration of Trust and Bylaws. The Articles of
Amendment and Restatement of Declaration of Trust, as amended, of EOP (the "EOP
Declaration of Trust") and the Bylaws of EOP (the "EOP Bylaws"), as in effect
immediately prior to the Effective Time of the Merger and, if approved by the
EOP shareholders, as amended by the Proposed EOP Charter Amendment Relating to
Domestically Controlled REIT Status (as defined herein) and the Proposed EOP
Charter Amendment Relating to Certain Voting Requirements (as defined herein),
shall continue in full force and effect after the Merger and, until further
amended in accordance with applicable Maryland law and, if approved by the EOP
shareholders, as amended by the Proposed EOP Charter Amendment Relating to
Domestically Controlled REIT Status (as defined herein) and the Proposed EOP
Charter Amendment Relating to Certain Voting Requirements (as defined herein).

     1.7 Trustees of EOP. The trustees of EOP following the Merger shall consist
of the trustees of EOP immediately prior to the Effective Time of the Merger,
who shall continue to serve for the balance of their unexpired terms or their
earlier death, resignation or removal, together with John S. Moody, William
Wilson III and Jan van der Vlist, each of whom shall, no later than the third
business day after the Effective Time of the Merger, become a trustee with terms
expiring in 2002, 2003 and 2003, respectively. Upon the expiration of the terms
of Mr. van der Vlist in 2003 and 2006, so long as PGGM and its Affiliates
continue to own in the aggregate 21,000,000 (as adjusted for stock splits,
reverse stock splits, stock dividends and similar actions) or more of the issued
and outstanding EOP Common Shares at all times up to the meeting of shareholders
at which trustees are being elected in such years, EOP shall take all action
necessary to nominate Mr. van der Vlist for re-election as a trustee of EOP for
an additional three-year term at any special or annual meeting of shareholders
at which trustees are being elected (or in connection with a written consent in
lieu of a meeting pursuant to which trustees are proposed to be elected). In the
event that Mr. Van der Vlist shall fail to stand for re-election as aforesaid
for any reason in either 2003 or 2006 or in the event of his earlier death or
resignation, and so long as PGGM and its Affiliates continue to own in the
aggregate 21,000,000 (as adjusted for stock splits, reverse stock splits, stock
dividends and similar actions) or more of the issued and outstanding EOP Common
Shares at such time, EOP shall take all action necessary to nominate a
replacement designated by PGGM, which replacement shall be subject to the
approval of EOP if such replacement is not an officer, director or employee of
PGGM, for election or re-election as a trustee of EOP for an additional
three-year term at

                                       A-4
<PAGE>   124

any special or annual meeting of shareholders at which trustees are being
elected (or in connection with a written consent in lieu of a meeting pursuant
to which trustees are proposed to be elected) or, in the case of a vacancy, at a
meeting of the Board of Trustees called for such purpose. Except as expressly
provided above in this Section 1.7, following their election as trustees, such
persons shall serve for their designated terms, subject to their earlier death,
resignation or removal.

     1.8 Effect on Shares. The effect of the Merger on the shares of capital
stock of Cornerstone shall be as provided in the Articles of Merger and in
Section 1.10 hereof. The Merger shall not change the shares of beneficial
interest of EOP outstanding immediately prior to the Merger.

     1.9 Effect on Partnership Interests. The effect of the Partnership Merger
on the partnership interests of Cornerstone Partnership shall be as provided in
the Certificate of Merger and in Section 1.10 hereof. The Partnership Merger
shall not change the partnership interests of EOP Partnership outstanding
immediately prior to the Merger.

     1.10 Exchange Ratios and Other Merger Consideration.

     (a) (i) The exchange ratio relating to the Partnership Merger shall be
0.7009 of a Class A Unit (as defined in the EOP Partnership Agreement) of EOP
Partnership ("EOP OP Unit"), for each Cornerstone OP Unit outstanding
immediately prior to the Effective Time of the Partnership Merger. The holders
of the EOP OP Units issued in the Partnership Merger (other than Cornerstone and
Subsidiaries (as defined herein) of Cornerstone) shall be entitled to redeem
such EOP OP Units immediately following the consummation of the Partnership
Merger (and thereafter) pursuant to the terms of the EOP Partnership Agreement,
except that for purposes of the exchange provisions thereof such EOP OP Units
shall be deemed to have been issued as of the date the related Cornerstone OP
Units were issued by Cornerstone Partnership (or if earlier, one year prior to
the Effective Time of the Partnership Merger), and shall be entitled to the same
rights and privileges as the holders of EOP OP Units outstanding on the date
hereof.

     (ii) The exchange ratio relating to the Partnership Merger shall be one
Class D Preferred Unit (as defined in the EOP Partnership Agreement), designated
a Class D 7.0% Cumulative Convertible Preferred Unit, of EOP Partnership ("EOP
Preferred OP Units"), for each Class A Partnership Preferred Unit (as defined in
the Cornerstone Partnership Agreement), designated a Class A 7% Cumulative
Convertible Preferred Unit of Cornerstone Partnership ("Cornerstone Preferred OP
Unit") outstanding immediately prior to the Effective Time of the Partnership
Merger. EOP, as the holder of the EOP Preferred OP Units issued in the
Partnership Merger, shall be entitled to the same rights and privileges as
Cornerstone, as the holder of the Cornerstone Preferred Units outstanding on the
date hereof.

     (b) The merger consideration to be paid to holders of capital stock of
Cornerstone in the Merger (collectively, the "Merger Consideration") is as
follows:

          (i) Each share of common stock with no par value of Cornerstone
     ("Cornerstone Common Stock") issued and outstanding immediately prior to
     the Effective Time of the Merger, which under the terms of Section 1.12 is
     to be converted into cash, shall be converted into the right to receive
     $18.00 in cash, without interest;

          (ii) Except as otherwise provided in Sections 1.10(b)(i) and 1.12, and
     subject to Section 1.15(g), each share of Cornerstone Common Stock issued
     and outstanding immediately prior to the Effective Time of the Merger
     (other than shares to be converted into the right to receive cash pursuant
     to Sections 1.10(b)(i) and 1.12) shall be converted into the right to
     receive 0.7009 of a validly issued, fully paid and nonassessable common
     share of beneficial interest, par value $.01 per share, of EOP (an "EOP
     Common Share") (the "Common Stock Exchange Ratio");

          (iii) Each share of 7.0% Cumulative Convertible Preferred Stock with
     no par value of Cornerstone ("Cornerstone 7% Preferred Stock") outstanding
     immediately prior to the Effective Time of the Merger shall be converted
     into the right to receive $18.00, together with accrued and unpaid
     dividends to the Effective Time of the Merger, in cash, without interest;
     and

                                       A-5
<PAGE>   125

          (iv) All such shares of Cornerstone Common Stock, when so converted as
     provided in Section 1.10(b)(i) or (ii), and all such shares of Cornerstone
     7% Preferred Stock, when so converted as provided in Section 1.10(b)(iii),
     shall no longer be outstanding and shall automatically be cancelled and
     retired and shall cease to exist, and each holder of a certificate (a
     "Certificate") theretofore representing any such shares shall cease to have
     any rights with respect thereto, except the right to receive, upon the
     surrender of such Certificate in accordance with Section 1.15(c), as
     applicable, (A) any dividends and other distributions in accordance with
     Section 1.15(d), (B) certificates representing the EOP Common Shares into
     which such shares of Cornerstone Common Stock are converted pursuant to
     Section 1.10(c)(ii) (if any), (C) cash into which such shares of
     Cornerstone Common Stock are converted pursuant to Section 1.10(c)(i), (D)
     cash into which such shares of Cornerstone 7% Preferred Stock are converted
     pursuant to Section 1.10(b)(iii), and (E) any cash, without interest, in
     lieu of fractional EOP Common Shares to be issued or paid in consideration
     for Cornerstone Common Stock upon the surrender of such Certificate in
     accordance with Sections 1.15(c) and 1.15(g).

     1.11 Election by Holders of Cornerstone Common Stock to Receive EOP Common
Shares or Cash. Each holder of shares of Cornerstone Common Stock (including,
without limitation, Limited Partners of Cornerstone Partnership who elect to
exercise, on a conditional or unconditional basis, the Redemption Right with
respect to all or a portion of their Cornerstone OP Units held by such Limited
Partners pursuant to Section 1.1(a) (with the Cornerstone OP Units with respect
to which such Exercise is made referred to as "Electing Cornerstone OP Units"))
shall have the right to submit a Form of Election specifying the number of
shares of Cornerstone Common Stock which such holder desires to have converted
into the right to receive EOP Common Shares in the Merger pursuant to Section
1.10(a)(ii) and the number which such holder desires to have converted into the
right to receive cash pursuant to Section 1.10(a)(i) in accordance with the
following procedures:

     (a) Each holder of shares of Cornerstone Common Stock and each holder of
Electing Cornerstone OP Units may specify in a request made in accordance with
the provisions of this Section 1.11 (an "Election") (i) the number of such
shares which such holder desires to have converted into the right to receive
cash in the Merger pursuant to Section 1.10(a)(i) (a "Cash Election") and (ii)
the number of such shares which such holder desires to have converted into the
right to receive EOP Common Shares in the Merger pursuant to Section 1.10(a)(ii)
(a "Share Election").

     (b) EOP and Cornerstone shall prepare, for use by stockholders of
Cornerstone (and each holder of Electing Cornerstone OP Units) in surrendering
Certificates representing shares of Cornerstone Common Stock, a form of election
(the "Form of Election") pursuant to which each holder of Cornerstone Common
Stock and each holder of Electing Cornerstone OP Units may make Elections. The
Form of Election shall be mailed to stockholders of record of Cornerstone as of
the record date for the Cornerstone Stockholders Meeting (as defined herein) and
to each holder of Cornerstone OP Units and shall accompany the Joint Proxy
Statement (as defined herein).

     (c) Cornerstone shall use commercially reasonable efforts to make the Form
of Election available to all persons who become stockholders of record of
Cornerstone and to all Limited Partners of Cornerstone Partnership during the
period between such record date and the second business day prior to the date of
the Cornerstone Stockholders Meeting, provided that only a Limited Partner of
Cornerstone Partnership who is a holder of Electing Cornerstone OP Units may
submit a Form of Election and such Form of Election shall apply only with
respect to shares of Cornerstone Common Stock issued to such Limited Partner
prior to the Effective Time of the Partnership Merger pursuant to the Redemption
Right as set forth in Section 1.1(a).

     (d) An Election shall have been properly made only if the Exchange Agent
(as defined herein) shall have received, by 5:00 p.m., Eastern Standard Time, on
the second business day (such time on such day being referred to herein as the
"Election Date") preceding the date of the Cornerstone Stockholders Meeting, a
Form of Election properly completed and signed (and not revoked) and accompanied
(in the case of holders of shares of Cornerstone Common Stock) by the
Certificate or Certificates representing the

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shares of Cornerstone Common Stock to which such Form of Election relates, duly
endorsed in blank or otherwise in form acceptable for transfer on the books of
Cornerstone (or by an appropriate guarantee of delivery of such Certificate or
Certificates as set forth in such Form of Election from a member of any
registered national securities exchange or of the National Association of
Securities Dealers, Inc. or a commercial bank or trust company having an office
or correspondent in the United States, provided such Certificate or Certificates
are in fact delivered by the time set forth in such guarantee of delivery).

     (e) Any holder of record of shares of Cornerstone Common Stock (and any
holder of Electing Cornerstone OP Units) may at any time prior to the Election
Date change such holder's Election by written notice received by the Exchange
Agent at or prior to the Election Date accompanied by a properly completed Form
of Election. EOP and Cornerstone shall have the right in their sole discretion
and by mutual agreement to permit changes in Elections after the Election Date.

     (f) Any holder of record of shares of Cornerstone Common Stock (and any
holder of Electing Cornerstone OP Units) may at any time prior to the Election
Date revoke such holder's Election by written notice received by the Exchange
Agent at or prior to the Election Date or by withdrawal prior to the Election
Date of such holder's Certificates previously deposited with the Exchange Agent.
Any revocation of an Election may be withdrawn by notice of such withdrawal
delivered at or prior to the Election Date. Any such holder who shall have
deposited Certificates with the Exchange Agent shall have the right to withdraw
such Certificates by written notice received by the Exchange Agent and thereby
revoke such holder's Election as of the Election Date at any time after the
expiration of the period of 60 days following the Election Date if the Merger
shall not have been consummated prior thereto. EOP shall obtain from the
Exchange Agent an agreement to return all Forms of Election and accompanying
Certificates to the stockholders submitting the same in the event this Agreement
shall be terminated in accordance with its terms.

     (g) EOP and Cornerstone by mutual agreement shall have the right to make
rules, not inconsistent with the terms of this Agreement, governing the validity
of Forms of Election, the manner and extent to which Elections are to be taken
into account in making the determinations prescribed by Section 1.12, the
issuance and delivery of certificates for EOP Common Shares into which shares of
Cornerstone Common Stock are converted in the Merger and the payment for shares
of Cornerstone Common Stock converted into the right to receive cash in the
Merger.

     1.12 Proration. The determination of whether shares of Cornerstone Common
Stock shall be converted in the Merger into EOP Common Shares in accordance with
the Common Stock Exchange Ratio or the right to receive $18.00 in cash shall be
made as set forth in this Section 1.12.

     (a) As is more fully set forth below, 58,551,525 shares of Cornerstone
Common Stock shall be converted in the Merger into the right to receive $18.00
per share in cash (which shares of Cornerstone Common Stock are referred to as
the "Cash Election Shares"), and all shares of Cornerstone Common Stock issued
and outstanding immediately prior to the Effective Time of the Merger in excess
of 58,551,525 shares shall be converted in the Merger into the right to receive
EOP Common Shares in accordance with the Common Stock Exchange Ratio (which
shares of Cornerstone Common Stock are referred to as the "Stock Election
Shares").

     (b) If Cash Elections are received for a number of shares of Cornerstone
Common Stock which is greater than 58,551,525 shares of Cornerstone Common
Stock, each Non-Electing Share (as defined herein) and each share of Cornerstone
Common Stock for which a Share Election has been received shall be converted in
the Merger into EOP Common Shares in accordance with the Common Stock Exchange
Ratio, and the shares of Cornerstone Common Stock for which Cash Elections have
been received shall be converted in the Merger into the right to receive cash
and EOP Common Shares in accordance with the Common Stock Exchange Ratio in the
following manner:

          each share of Cornerstone Common Stock covered by a Cash Election
     shall be converted into the right to receive (i) an amount in cash, without
     interest, equal to the product of (x) $18.00 and (y) a fraction (the "Cash
     Fraction") the numerator of which shall be 58,551,525 and the

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     denominator of which shall be the aggregate number of shares of Cornerstone
     Common Stock covered by all Cash Elections, and (ii) a number of EOP Common
     Shares equal to the product of (x) the Common Stock Exchange Ratio and (y)
     a fraction equal to one minus the Cash Fraction.

     (c) If Share Elections are received for a number of shares of Cornerstone
Common Stock which is greater than the total number of Stock Election Shares
issued and outstanding immediately prior to the Effective Time of the Merger,
each Non-Electing Share and each share of Cornerstone Common Stock for which a
Cash Election has been received shall be converted in the Merger into the right
to receive $18.00 in cash, without interest, and the shares of Cornerstone
Common Stock for which Share Elections have been received shall be converted in
the Merger into EOP Shares in accordance with the Common Stock Exchange Ratio
and the right to receive cash in the following manner:

          each share of Cornerstone Common Stock covered by a Share Election
     shall be converted into the right to receive (i) a number of EOP Common
     Shares equal to the product of (x) the Common Stock Exchange Ratio and (y)
     a fraction (the "Stock Fraction"), the numerator of which shall be a number
     equal to the total number of Stock Election Shares issued and outstanding
     immediately prior to the Effective Time of the Merger and the denominator
     of which shall be the aggregate number of shares of Cornerstone Common
     Stock covered by all Share Elections, and (ii) an amount in cash, without
     interest, equal to the product of (x) $18.00 and (y) a fraction equal to
     one minus the Stock Fraction.

     (d) If Non-Electing Shares (as defined herein) are not converted under
either Section 1.12(b) or Section 1.12(c), then each Non-Electing Share shall be
converted into the right to receive EOP Common Shares and the right to receive
cash on a proportionate basis, relative to all other Non-Electing Shares, so
that the aggregate number of shares of Cornerstone Common Stock converted into
the right to receive EOP Common Shares equals, as closely as possible, the
number of Stock Election Shares, and the aggregate number of shares of
Cornerstone Common Stock converted into the right to receive cash equals, as
closely as possible, the number of Cash Election Shares.

     (e) For purposes of this Section 1.12, outstanding shares of Cornerstone as
to which an election is not in effect at the Election Date and shares as to
which an Election has been withdrawn after the 60-day period following the
Election Date and prior to the Effective Time of the Merger shall be called
"Non-Electing Shares." If EOP and Cornerstone shall determine for any reason
that any Election was not properly made with respect to shares of Cornerstone
Common Stock, such Election shall be deemed ineffective and shares of
Cornerstone Common Stock covered by such Election shall, for purposes hereof, be
deemed to be Non-Electing Shares.

     1.13 Partner Approval. Cornerstone shall seek the requisite approval of the
partners of Cornerstone Partnership of the Merger, the withdrawal of Cornerstone
as general partner and the Partnership Merger to the extent required by the
Cornerstone Partnership Agreement to effectuate the transactions contemplated by
this Agreement (collectively, the "Cornerstone Partner Approvals"). EOP shall
seek the requisite approval of the partners of EOP Partnership of the Merger and
the Partnership Merger to the extent required by the EOP Partnership Agreement
to effectuate the transactions contemplated by this Agreement (collectively, the
"EOP Partner Approvals," and together with the Cornerstone Partner Approvals,
the "Partner Approvals").

     1.14 No Appraisal Rights. The holders of Cornerstone Common Stock,
Cornerstone OP Units, EOP Common Shares and EOP OP Units are not entitled under
applicable law to appraisal or similar rights as a result of the Mergers and, in
the case of the holders of the Cornerstone 7% Preferred Stock, such holders
irrevocably have waived all such rights.

     1.15 Exchange of Certificates; Pre-Closing Dividends; Fractional Shares.

     (a) Exchange Agent. Prior to the Effective Time, EOP shall appoint
Equiserve LLC as the exchange agent, or another bank or trust company reasonably
acceptable to Cornerstone, to act as exchange agent (the "Exchange Agent") for
the exchange of the Merger Consideration upon surrender of certificates

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representing issued and outstanding shares of Cornerstone Common Stock and
Cornerstone 7% Preferred Stock.

     (b) EOP to Provide Merger Consideration. EOP shall provide to the Exchange
Agent on or before the Effective Time of the Merger, for the benefit of the
holders of Cornerstone Common Stock and Cornerstone 7% Preferred Stock, the
Merger Consideration issuable in exchange for the issued and outstanding
Cornerstone Common Stock and Cornerstone 7% Preferred Stock pursuant to Section
1.10, together with any cash required to make payments in lieu of any fractional
shares pursuant to Section 1.15(g) (the "Exchange Fund"). The Exchange Agent
shall invest any cash included in the Exchange Fund as directed by EOP, on a
daily basis. Any interest or other income resulting from such investments shall
be paid to EOP. Cornerstone shall provide to the Exchange Agent not later than
one business day prior to the Effective Time of the Merger, for the benefit of
the holders of Cornerstone Common Stock, cash payable in respect of any
dividends required pursuant to Section 1.15(d)(i) or (ii). Such cash shall be
invested in accordance with written directions delivered by Cornerstone to the
Exchange Agent not later than one business day prior to the Effective Time of
the Merger, with any cash earned on such investments to be paid to EOP as the
successor to Cornerstone in the Merger. EOP shall use commercially reasonable
efforts to cause the Exchange Agent to mail the Merger Consideration to the
holders of Cornerstone Common Stock (including to holders of Electing
Cornerstone OP Units who made an unconditional Exercise of the Redemption Right
or who made a conditional exercise thereof and who will receive cash in the
Merger, in each case who have properly submitted a Form of Election prior to the
Election Date) and Cornerstone 7% Preferred Stock not later than five business
days after the Effective Time of the Merger.

     (c) Exchange Procedure. As soon as reasonably practicable after the
Effective Time, EOP shall use commercially reasonable efforts to cause the
Exchange Agent to mail to each holder of record of a Certificate or Certificates
which immediately prior to the Effective Time represented outstanding shares of
Cornerstone Common Stock (other than to holders of Cornerstone Common Stock who
previously surrendered with their Forms of Election their Certificates for
Cornerstone Common Stock) or Cornerstone 7% Preferred Stock Certificate whose
shares were converted into the right to receive the Merger Consideration
pursuant to Section 1.10, (i) a letter of transmittal (which shall specify that
delivery shall be effected, and risk of loss and title to the Certificates shall
pass, only upon delivery of the Certificates to the Exchange Agent and shall be
in a form and have such other provisions as EOP may reasonably specify) and (ii)
instructions for use in effecting the surrender of the Certificates in exchange
for the Merger Consideration. To the extent not previously surrendered with a
Form of Election, upon surrender of a Certificate for cancellation to the
Exchange Agent or to such other agent or agents as may be appointed by EOP,
together with such letter of transmittal, duly executed, and such other
documents as may reasonably be required by the Exchange Agent, the holder of
such Certificate shall be entitled to receive in exchange therefor the Merger
Consideration into which the shares of Cornerstone Common Stock or Cornerstone
7% Preferred Stock, as applicable, theretofore represented by such Certificate
shall have been converted pursuant to Section 1.10, together with any dividends
or other distributions to which such holder is entitled pursuant to Section
1.15(d) and cash, if any, payable in lieu of fractional shares pursuant to
Section 1.15(g), to be mailed (or made available for collection by hand if so
elected by the surrendering holder) within five business days of receipt
thereof, and the Certificate so surrendered shall forthwith be canceled. In the
event of a transfer of ownership of shares of Cornerstone Common Stock or
Cornerstone 7% Preferred Stock which is not registered in the transfer records
of Cornerstone, payment may be made to a person other than the person in whose
name the Certificate so surrendered is registered if such Certificate shall be
properly endorsed or otherwise be in proper form for transfer and the person
requesting such payment either shall pay any transfer or other taxes required by
reason of such payment being made to a person other than the registered holder
of such Certificate or establish to the satisfaction of EOP that such tax or
taxes have been paid or are not applicable. Until surrendered as contemplated by
this Section 1.15, each Certificate shall be deemed at any time after the
Effective Time to represent only the right to receive upon such surrender the
Merger Consideration, without interest, into which the shares of Cornerstone
Common Stock or Cornerstone 7% Preferred Stock heretofore represented by such
Certificate shall have been converted pursuant to Section 1.10, and any
dividends or other distributions to
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which such holder is entitled pursuant to Section 1.15(d). No interest will be
paid or will accrue on the Merger Consideration upon the surrender of any
Certificate or on any cash payable pursuant to Section 1.15(d) or Section
1.15(g). EOP or the Exchange Agent, as applicable, shall be entitled, in its
sole and absolute discretion, to deduct and withhold from the cash or EOP Common
Shares, or any combination thereof, that otherwise is payable pursuant to this
Agreement to any holder of shares of Cornerstone Common Stock or Cornerstone 7%
Preferred Stock such amounts as EOP or the Exchange Agent is required to deduct
and withhold with respect to the making of such payment under the Code or under
any provision of state, local or foreign tax law, provided that in determining
whether withholding under Section 1445 of the Code is required, EOP shall take
into account (and shall request the Exchange Agent to take into account) Section
1445(b)(6) of the Code and Treasury Regulations Section 1.1445-2(c)(2). For this
purpose, any EOP Common Shares deducted and withheld by EOP shall be valued at
the last trading price of the EOP Common Shares on the New York Stock Exchange
on the Effective Date of the Merger. To the extent that amounts are so withheld
by EOP or the Exchange Agent, such withheld amounts shall be treated for all
purposes of this Agreement as having been paid to the holder of the shares of
Cornerstone Common Stock or Cornerstone 7% Preferred Stock, as applicable, in
respect of which such deduction and withholding was made by EOP or the Exchange
Agent.

     (d) Record Dates for Final Dividends; Distributions with Respect to
Unexchanged Shares.

          (i) To the extent necessary to satisfy the requirements of Section
     857(a)(1) of the Code for the taxable year of Cornerstone ending at the
     Effective Time of the Merger (and avoid the payment of tax with respect to
     undistributed income), Cornerstone shall declare a dividend (the "Final
     Company Dividend") to holders of shares of Cornerstone Common Stock and
     Cornerstone 7% Preferred Stock, the record date for which shall be the
     close of business on the last business day prior to the Effective Time of
     the Merger, in an amount equal to the minimum dividend sufficient to permit
     Cornerstone to satisfy such requirements. If Cornerstone determines it is
     necessary to declare the Final Company Dividend, it shall notify EOP at
     least 10 days prior to the date for the Cornerstone Stockholders Meeting,
     and EOP shall declare a dividend per share to holders of shares of EOP
     Common Shares, the record date for which shall be the close of business on
     the last business day prior to the Effective Time, in an amount per EOP
     Common Share equal to the quotient obtained by dividing (x) the Final
     Company Dividend paid by Cornerstone with respect to each share of
     Cornerstone Common Stock by (y) the Common Stock Exchange Ratio. The
     dividends payable hereunder to holders of Cornerstone Common Stock and
     Cornerstone 7% Preferred Stock shall be paid on the last business day
     immediately preceding the Closing Date.

          (ii) No dividends or other distributions with respect to EOP Common
     Shares with a record date after the Effective Time shall be paid to the
     holder of any unsurrendered Certificate with respect to the shares of EOP
     Common Shares represented thereby, and no cash payment in lieu of
     fractional shares shall be paid to any such holder pursuant to Section
     1.15(g), in each case until the surrender of such Certificate in accordance
     with this Section 1.15. Subject to the effect of applicable escheat laws,
     following surrender of any such Certificate there shall be paid to the
     holder of such Certificate, without interest, (i) at the time of such
     surrender, the amount of any cash payable pursuant to Section 1.10 and/or
     in lieu of any fractional EOP Common Shares to which such holder is
     entitled pursuant to Section 1.15(g) and (ii) if such Certificate is
     exchangeable for one or more whole EOP Common Shares, (x) at the time of
     such surrender the amount of dividends or other distributions with a record
     date after the Effective Time theretofore paid with respect to such whole
     EOP Common Shares and (y) at the appropriate payment date, the amount of
     dividends or other distributions with a record date after the Effective
     Time but prior to such surrender and with a payment date subsequent to such
     surrender payable with respect to such whole EOP Common Shares.

     (e) No Further Ownership Rights in Cornerstone Common Stock and Cornerstone
7% Preferred Stock. All Merger Consideration paid upon the surrender of
Certificates in accordance with the terms of this Section 1.15 (including any
cash paid pursuant to Section 1.15(g)) shall be deemed to have been paid in full
satisfaction of all rights pertaining to the Cornerstone Common Stock and
Cornerstone 7% Preferred Stock, as applicable, theretofore represented by such
Certificates; provided, however, that
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Cornerstone shall transfer to the Exchange Agent cash sufficient to pay any
dividends or make any other distributions with a record date prior to the
Effective Time which may have been declared or made by Cornerstone on such
Cornerstone Common Stock or Cornerstone 7% Preferred Stock in accordance with
the terms of this Agreement or prior to the date of this Agreement and which
remain unpaid at the Effective Time and have not been paid prior to such
surrender, and there shall be no further registration of transfers on the stock
transfer books of Cornerstone of the Cornerstone Common Stock or Cornerstone 7%
Preferred Stock which were outstanding immediately prior to the Effective Time.
If, after the Effective Time, Certificates are presented to EOP for any reason,
they shall be canceled and exchanged as provided in this Section 1.15.

     (f) No Liability. None of Cornerstone, EOP or the Exchange Agent shall be
liable to any person in respect of any Merger Consideration or dividends
delivered to a public official pursuant to any applicable abandoned property,
escheat or similar law. Any portion of the Exchange Fund delivered to the
Exchange Agent pursuant to this Agreement that remains unclaimed for 12 months
after the Effective Time shall be redelivered by the Exchange Agent to EOP, upon
demand, and any holders of Certificates who have not theretofore complied with
Section 1.15(c) shall thereafter look only to EOP for delivery of the Merger
Consideration and any unpaid dividends, subject to applicable escheat and other
similar laws.

     (g) No Fractional Shares

          (i) No certificates or scrip representing fractional EOP Common Shares
     shall be issued upon the surrender for exchange of Certificates, and such
     fractional share interests will not entitle the owner thereof to vote, to
     receive dividends or to any other rights of a shareholder of EOP.

          (ii) No fractional EOP Common Shares shall be issued pursuant to this
     Agreement. In lieu of the issuance of any fractional EOP Common Shares
     pursuant to this Agreement, each holder of Cornerstone Common Stock upon
     surrender of a Certificate for exchange shall be paid an amount in cash
     (without interest), rounded to the nearest cent, determined by multiplying
     (i) the average closing price of one EOP Common Share on the New York Stock
     Exchange on the five trading days immediately preceding the Closing Date by
     (ii) the fractional amount of 0.7009 of an EOP Common Share which such
     holder would otherwise be entitled to receive under this Section 1.15.

     (h) Lost Certificates. If any Certificate shall have been lost, stolen or
destroyed, upon the making of an affidavit of that fact by the person claiming
such Certificate to be lost, stolen or destroyed and, if required by EOP or the
Exchange Agent, the posting by such person of a bond in such reasonable amount
as EOP or the Exchange Agent may direct (but consistent with the practices EOP
applies to its own shareholders) as indemnity against any claim that may be made
against them with respect to such Certificate, the Exchange Agent will issue in
exchange for such lost, stolen or destroyed Certificate the EOP Common Shares or
cash to which the holders thereof are entitled pursuant to Section 1.10, any
cash payable pursuant to Section 1.15(g) to which the holders thereof are
entitled and any dividends or other distributions to which the holders thereof
are entitled pursuant to Section 1.15(d).

     (i) Applicability to Partnership Merger. Except for the provisions relating
to the Exchange Agent, certificates and the exchange procedure (which shall not
be applicable), all other provisions of this Section 1.15 shall apply to
Cornerstone Partnership, EOP Partnership, the Cornerstone OP Units and the
Cornerstone OP Preferred Units with respect to the Partnership Merger.

                                   ARTICLE 2

                 REPRESENTATIONS AND WARRANTIES OF CORNERSTONE
                          AND CORNERSTONE PARTNERSHIP

     Except as set forth in the Cornerstone SEC Documents (as defined herein) or
in the letter of even date herewith signed by the President and Chief Executive
Officer or the Chief Operating Officer of

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Cornerstone and delivered to EOP prior to the execution hereof (the "Cornerstone
Disclosure Letter"), Cornerstone and Cornerstone Partnership represent and
warrant to EOP and EOP Partnership as follows:

     2.1 Organization, Standing and Power. Cornerstone has been duly organized
and is validly existing and in good standing under the laws of the State of
Nevada. Cornerstone has all requisite corporate power and authority to own,
operate, lease and encumber its properties and carry on its business as now
being conducted. The Cornerstone Restated Articles of Incorporation, as amended
(the "Cornerstone Articles") are in effect, and no dissolution, revocation or
forfeiture proceedings regarding Cornerstone have been commenced. Cornerstone is
duly qualified or licensed to do business as a foreign corporation and is in
good standing in each jurisdiction in which the nature of its business or the
ownership or leasing of its properties makes such qualification or licensing
necessary, other than in such jurisdictions where the failure to be so qualified
or licensed, individually or in the aggregate, would not have a material adverse
effect on the business, properties, assets, financial condition or results of
operations of Cornerstone, Cornerstone Partnership and the Cornerstone
Subsidiaries (as defined herein), taken as a whole (a "Cornerstone Material
Adverse Effect"). Cornerstone has delivered to EOP complete and correct copies
of the Cornerstone Articles and Cornerstone's Amended and Restated Bylaws (the
"Cornerstone Bylaws"), in each case, as amended or supplemented to the date of
this Agreement.

     2.2 Cornerstone Subsidiaries.

     (a) Schedule 2.2 to the Cornerstone Disclosure Letter sets forth (i) each
Subsidiary (as defined herein) of Cornerstone (the "Cornerstone Subsidiaries")
and the Cornerstone Non-controlled Subsidiary (which Cornerstone Non-controlled
Subsidiary constitutes the only entity in which Cornerstone owns a non-voting
equity interest and has no right to control except as set forth on Schedule 2.4
of the Cornerstone Disclosure Letter), (ii) the ownership interest therein of
Cornerstone, (iii) if not, directly or indirectly, wholly owned by Cornerstone,
the identity and ownership interest of each of the other owners of such
Cornerstone Subsidiary or Cornerstone Non-controlled Subsidiary, as applicable,
(iv) each office property and other commercial property owned by such
Cornerstone Subsidiary or Cornerstone Non-controlled Subsidiary, as applicable,
and (v) if not wholly owned by such Cornerstone Subsidiary or Cornerstone
Non-controlled Subsidiary, as applicable, the identity and ownership interest of
each of the other owners of such property. As used in this Agreement,
"Subsidiary" of any Person (as defined herein) means any corporation,
partnership, limited liability company, joint venture, trust or other legal
entity of which such Person owns (either directly or through or together with
another Subsidiary of such Person) either (i) a general partner, managing member
or other similar interest, or (ii)(A) 10% or more of the voting power of the
voting capital stock or other equity interests, or (B) 10% or more of the
outstanding voting capital stock or other voting equity interests of such
corporation, partnership, limited liability company, joint venture or other
legal entity. As used herein, "Person" means an individual, corporation,
partnership, limited liability company, joint venture, association, trust,
unincorporated organization or other entity. Schedule 2.4 of the Cornerstone
Disclosure Letter sets forth a true and complete list of the equity securities
owned by Cornerstone, directly or indirectly, in any corporation, partnership,
limited liability company, joint venture or other legal entity, excluding
Cornerstone Subsidiaries and the Cornerstone Non-controlled Subsidiary.

     (b) Except as set forth in Schedule 2.2 to the Cornerstone Disclosure
Letter, (i) all of the outstanding shares of capital stock of each Cornerstone
Subsidiary and Cornerstone Non-controlled Subsidiary that is a corporation have
been duly authorized, validly issued and are (A) fully paid and nonassessable
and not subject to preemptive rights, (B) owned by Cornerstone or by another
Cornerstone Subsidiary or Cornerstone Non-controlled Subsidiary and (C) owned
free and clear of all pledges, claims, liens, charges, encumbrances and security
interests of any kind or nature whatsoever (collectively, "Liens") and (ii) all
equity interests in each Cornerstone Subsidiary that is a partnership, joint
venture, limited liability company or trust which are owned by Cornerstone, by
another Cornerstone Subsidiary or Cornerstone Non-controlled Subsidiary or by
Cornerstone and another Cornerstone Subsidiary or Cornerstone Non-controlled
Subsidiary are owned free and clear of all Liens other than pledges, if any,
contained in organizational documents of such Cornerstone Subsidiary and given
to secure performance thereunder. Each Cornerstone Subsidiary and Cornerstone
Non-controlled Subsidiary that is a corporation
                                      A-12
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is duly incorporated, validly existing and in good standing under the laws of
its jurisdiction of incorporation and has the requisite corporate power and
authority to own, operate, lease and encumber its properties and carry on its
business as now being conducted, and each Cornerstone Subsidiary that is a
partnership, limited liability company or trust is duly organized, validly
existing and in good standing under the laws of its jurisdiction of organization
and has the requisite power and authority to own, operate, lease and encumber
its properties and carry on its business as now being conducted. Each
Cornerstone Subsidiary and Cornerstone Non-controlled Subsidiary is duly
qualified or licensed to do business and is in good standing in each
jurisdiction in which the nature of its business or the ownership or leasing of
its properties makes such qualification or licensing necessary, other than in
such jurisdictions where the failure to be so qualified or licensed,
individually or in the aggregate, would not reasonably be expected to have a
Cornerstone Material Adverse Effect. Complete and correct copies of the Articles
of Incorporation, Bylaws, organization documents and partnership, joint venture
and operating agreements of each Cornerstone Subsidiary and Cornerstone
Non-controlled Subsidiary, as amended to the date of this Agreement, have been
previously delivered or made available to EOP. No effective amendment has been
made to the Cornerstone Partnership Agreement since January 21, 2000.

     2.3 Capital Structure.

     (a) The authorized shares of capital stock of Cornerstone consist of
250,000,000 shares of Cornerstone Common Stock, 129,638,245 of which are issued
and 129,280,012 of which are outstanding on the date of this Agreement, and
65,000,000 shares of preferred stock with no par value, of which 3,030,303
shares of Cornerstone 7% Preferred Stock are issued and outstanding and 344,828
shares of Redeemable Preferred Stock are designated with none outstanding on the
date of this Agreement.

     (b) Set forth in Schedule 2.3(b) to the Cornerstone Disclosure Letter is a
true and complete list of the following: (i) each qualified or nonqualified
option to purchase shares of Common Stock granted under Cornerstone's 1998
Long-Term Incentive Plan or any other formal or informal arrangement
(collectively, the "Cornerstone Stock Options"); and (ii) all other warrants or
other rights to acquire Cornerstone Common Stock, all stock appreciation rights,
restricted stock, dividend equivalents, deferred compensation accounts,
performance awards, restricted stock unit awards and other awards which are
outstanding on the date of this Agreement ("Cornerstone Stock Rights"). Schedule
2.3(b) to the Cornerstone Disclosure Letter sets forth for each Cornerstone
Stock Option and Cornerstone Stock Right the name of the grantee, the date of
the grant, the number of shares of Cornerstone Common Stock subject to each
option or other award, and the exercise price per share. On the date of this
Agreement, except as set forth in this Section 2.3 or in Schedule 2.3(b) to the
Cornerstone Disclosure Letter, no shares of Cornerstone Common Stock were
outstanding or reserved for issuance.

     (c) All outstanding shares of Cornerstone Common Stock are duly authorized,
validly issued, fully paid and nonassessable and not subject to preemptive
rights. There are no bonds, debentures, notes or other indebtedness of
Cornerstone having the right to vote (or convertible into, or exchangeable for,
securities having the right to vote) on any matters on which shareholders of
Cornerstone may vote, except for the Cornerstone convertible promissory note
dated as of January 1, 1996 which does not have voting rights but is convertible
into Cornerstone Common Stock (the "Cornerstone Convertible Promissory Note").

     (d) Other than (i) as set forth in this Section 2.3 or in Schedule 2.3(b),
2.3(c), or 2.3(d) to the Cornerstone Disclosure Letter, (ii) Cornerstone OP
Units, which may be redeemed for cash or, at the option of Cornerstone,
Cornerstone Common Stock at a rate of one share of Cornerstone Common Stock for
each Cornerstone OP Unit, (iii) shares of Cornerstone Common Stock issuable upon
the conversion of Cornerstone 7% Preferred Stock, and (iv) shares of Cornerstone
Common Stock issuable pursuant to the Cornerstone Convertible Promissory Note,
as of the date of this Agreement, there are no outstanding securities, options,
warrants, calls, rights, commitments, agreements, arrangements or undertakings
of any kind to which Cornerstone or any Cornerstone Subsidiary or Cornerstone
Non-controlled Subsidiary is a party or by which such entity is bound,
obligating Cornerstone or any Cornerstone Subsidiary or Cornerstone
Non-controlled Subsidiary to issue, deliver or sell, or cause to be issued,
delivered or sold,

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additional shares of capital stock, voting securities or other ownership
interests of Cornerstone or any Cornerstone Subsidiary or Cornerstone
Non-controlled Subsidiary or obligating Cornerstone or any Cornerstone
Subsidiary or Cornerstone Non-controlled Subsidiary to issue, grant, extend or
enter into any such security, option, warrant, call, right, commitment,
agreement, arrangement or undertaking (other than to Cornerstone or a
Cornerstone Subsidiary or a Cornerstone Non-controlled Subsidiary).

     (e) As of the date of this Agreement, 148,735,829 Cornerstone OP Units are
validly issued and outstanding, fully paid and nonassessable and not subject to
preemptive rights, of which 129,638,245 are owned by Cornerstone and 3,030,303
Cornerstone Preferred OP Units are validly issued and outstanding, fully paid
and nonassessable and not subject to preemptive rights, all of which are owned
by Cornerstone. Schedule 2.3(e) to the Cornerstone Disclosure Schedule sets
forth the name of each holder of Cornerstone OP Units and the number of
Cornerstone OP Units owned by each such holder as of the date of this Agreement.
Except as provided in the Cornerstone Partnership Agreement or as set forth on
Schedule 2.3(d), Cornerstone Partnership has not issued or granted and is not a
party to any outstanding commitments of any kind relating to, or any presently
effective agreements or understandings with respect to, the issuance or sale of
interests in Cornerstone Partnership, whether issued or unissued, or securities
convertible or exchangeable into interests in Cornerstone Partnership.

     (f) Except for the distribution of $0.31 per share of Cornerstone Common
Stock and per Cornerstone OP Unit declared on January 20, 2000, and payable on
February 29, 2000, to holders of record on January 31, 2000, of shares of
Cornerstone Common Stock and Cornerstone OP Units, all dividends on Cornerstone
Common Stock and Cornerstone 7% Preferred Stock and all distributions on
Cornerstone OP Units, which have been declared prior to the date of this
Agreement, have been paid in full.

     (g) Set forth on Schedule 2.3(g) to the Cornerstone Disclosure Letter is a
list of each registration rights agreement or other agreement between
Cornerstone and/or Cornerstone Partnership, on the one hand, and one or more
other parties, on the other hand, which sets forth the rights of any such other
party or parties to cause the registration of any securities of Cornerstone
and/or Cornerstone Partnership pursuant to the Securities Act of 1933, as
amended (the "Securities Act").

     2.4 Other Interests. Except for interests in the Cornerstone Subsidiaries,
the Cornerstone Non-controlled Subsidiary and certain other entities as set
forth in Schedule 2.4 to the Cornerstone Disclosure Letter (the "Cornerstone
Other Interests"), none of Cornerstone, Cornerstone Partnership, any Cornerstone
Subsidiary or the Cornerstone Non-controlled Subsidiary owns directly or
indirectly any interest or investment (whether equity or debt) in any
corporation, partnership, joint venture, business, trust, limited liability
company or other entity (other than investments in short-term investment
securities). With respect to the Cornerstone Other Interests, Cornerstone
Partnership owns such interests free and clear of all Liens other than pledges,
if any, contained in organizational documents of such Cornerstone Other
Interests and given to secure performance thereunder. None of Cornerstone,
Cornerstone Partnership, any Cornerstone Subsidiary or the Cornerstone
Non-controlled Subsidiary is in material breach of any provision of any
agreement, document or contract which is of a material nature governing its
rights in or to the Cornerstone Other Interests, all of which agreements,
documents and contracts are (a) listed in Schedule 2.4 to the Cornerstone
Disclosure Letter, (b) unmodified except as described therein and (c) in full
force and effect. To the Knowledge of Cornerstone (as defined herein), the other
parties to any such agreement, document or contract which is of a material
nature are not in material breach of any of their respective obligations under
such agreements, documents or contracts.

     2.5 Authority; Noncontravention; Consents.

     (a) Cornerstone has the requisite corporate power and authority to enter
into this Agreement and, subject to the requisite Cornerstone stockholder
approval of the Merger Agreement and the Merger and any other matters reasonably
and timely requested by any other party to effectuate the transactions
contemplated by this Agreement (collectively, the "Cornerstone Stockholder
Approvals"), to consummate the transactions contemplated by this Agreement to
which Cornerstone is a party. The execution and delivery of this Agreement by
Cornerstone and the consummation by Cornerstone of the transactions contemplated
by this Agreement to which Cornerstone is a party have been duly authorized by
all
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necessary action on the part of Cornerstone, except for and subject to the
Cornerstone Stockholder Approvals and the Cornerstone Partner Approvals. This
Agreement has been duly executed and delivered by Cornerstone and constitutes a
valid and binding obligation of Cornerstone, enforceable against Cornerstone in
accordance with and subject to its terms, subject to applicable bankruptcy,
insolvency, moratorium or other similar laws relating to creditors' rights and
general principles of equity.

     (b) Cornerstone Partnership has the requisite partnership power and
authority to enter into this Agreement and, subject to the requisite Cornerstone
Partner Approvals, to consummate the transactions contemplated by this Agreement
to which Cornerstone Partnership is a party. The execution and delivery of this
Agreement by Cornerstone Partnership and the consummation by Cornerstone
Partnership of the transactions contemplated by this Agreement to which
Cornerstone Partnership is a party have been duly authorized by all necessary
action on the part of Cornerstone Partnership, except for and subject to the
Cornerstone Stockholder Approvals and the Cornerstone Partner Approvals. This
Agreement has been duly executed and delivered by Cornerstone Partnership and
constitutes a valid and binding obligation of Cornerstone Partnership,
enforceable against Cornerstone Partnership in accordance with and subject to
its terms, subject to applicable bankruptcy, insolvency, moratorium or other
similar laws relating to creditors' rights and general principles of equity.

     (c) Except as set forth in Schedule 2.5(c)(1) to the Cornerstone Disclosure
Letter, the execution and delivery of this Agreement by Cornerstone do not, and
the consummation of the transactions contemplated by this Agreement to which
Cornerstone is a party and compliance by Cornerstone with the provisions of this
Agreement will not, conflict with, or result in any violation of, or default
(with or without notice or lapse of time, or both) under, or give rise to a
right of termination, cancellation or acceleration of any material obligation or
to material loss of a benefit under, or result in the creation of any Lien upon
any of the properties or assets of Cornerstone or any Cornerstone Subsidiary
under, (i) the Cornerstone Articles or Cornerstone Bylaws or the comparable
charter or organizational documents or partnership, operating, or similar
agreement (as the case may be) of any Cornerstone Subsidiary or Cornerstone
Non-controlled Subsidiary, each as amended or supplemented, (ii) any loan or
credit agreement, note, bond, mortgage, indenture, merger or other acquisition
agreement, reciprocal easement agreement, lease or other agreement, instrument,
permit, concession, franchise or license applicable to Cornerstone or any
Cornerstone Subsidiary or Cornerstone Non-controlled Subsidiary or their
respective properties or assets or (iii) subject to the governmental filings and
other matters referred to in the following sentence, any judgment, order,
decree, statute, law, ordinance, rule or regulation (collectively, "Laws")
applicable to Cornerstone or any Cornerstone Subsidiary or Cornerstone
Non-controlled Subsidiary, or their respective properties or assets, other than,
in the case of clause (ii) or (iii), any such conflicts, violations, defaults,
rights, loss or Liens that individually or in the aggregate would not (x) have a
Cornerstone Material Adverse Effect or (y) prevent the consummation of the
transactions contemplated by this Agreement. No consent, approval, order or
authorization of, or registration, declaration or filing with, any federal,
state or local government or any court, administrative or regulatory agency or
commission or other governmental authority or agency, domestic or foreign (a
"Governmental Entity"), is required by or with respect to Cornerstone or any
Cornerstone Subsidiary or Cornerstone Non-controlled Subsidiary in connection
with the execution and delivery of this Agreement by Cornerstone or the
consummation by Cornerstone of the transactions contemplated by this Agreement,
except for (i) the filing with the Securities and Exchange Commission (the
"SEC") of the Joint Proxy Statement (as defined herein), (ii) the acceptance for
record of the Maryland Articles of Merger by the Department, (iii) the filing of
the Nevada Articles of Merger with the Secretary of State of the State of
Nevada, (iv) the filing of the Certificate of Merger with the Office of the
Secretary of State of the State of Delaware and (v) such other consents,
approvals, orders, authorizations, registrations, declarations and filings (A)
as are set forth in Schedule 2.5(c)(2) to the Cornerstone Disclosure Letter, (B)
as may be required under (w) the Hart-Scott-Rodino Antitrust Improvements Act of
1976, as amended (the "HSR Act"), (x) laws requiring transfer, recordation or
gains tax filings, (y) federal, state or local environmental laws or (z) the
"blue sky" laws of various states, to the extent applicable or (C) which, if not
obtained or made, would not prevent or delay in any material respect the
consummation of any of the transactions contemplated by this Agreement or
otherwise prevent Cornerstone from performing its obligations under
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this Agreement in any material respect or reasonably be expected to have,
individually or in the aggregate, a Cornerstone Material Adverse Effect.

     2.6 SEC Documents; Financial Statements; Undisclosed
Liabilities.  Cornerstone has filed all required reports, schedules, forms,
statements and other documents with the SEC since January 1, 1997 through the
date hereof (the "Cornerstone SEC Documents"). Schedule 2.6(a) to the
Cornerstone Disclosure Letter contains a complete list of all Cornerstone SEC
Documents filed by Cornerstone with the SEC under the Securities Exchange Act of
1934, as amended (the "Exchange Act"), between January 1, 1997 and the date of
this Agreement. All of the Cornerstone SEC Documents (other than preliminary
material), as of their respective filing dates, complied in all material
respects with all applicable requirements of the Securities Act and the Exchange
Act and, in each case, the rules and regulations promulgated thereunder
applicable to such Cornerstone SEC Documents. None of the Cornerstone SEC
Documents at the time of filing contained any untrue statement of a material
fact or omitted to state any material fact required to be stated therein or
necessary in order to make the statements therein, in light of the circumstances
under which they were made, not misleading, except to the extent such statements
have been modified or superseded by later Cornerstone SEC Documents filed and
publicly available prior to the date of this Agreement. The consolidated
financial statements of Cornerstone included in the Cornerstone SEC Documents
complied as to form in all material respects with applicable accounting
requirements and the published rules and regulations of the SEC with respect
thereto, have been prepared in accordance with generally accepted accounting
principles ("GAAP") (except, in the case of unaudited statements, as permitted
by the applicable rules and regulations of the SEC) applied on a consistent
basis during the periods involved (except as may be indicated in the notes
thereto) and fairly presented in all material respects, in accordance with the
applicable requirements of GAAP and the applicable rules and regulations of the
SEC, the consolidated financial position of Cornerstone and its Subsidiaries
taken as a whole, as of the dates thereof and the consolidated results of
operations and cash flows for the periods then ended (subject, in the case of
unaudited statements, to normal year-end audit adjustments). Except as set forth
in Schedule 2.6(b) to the Cornerstone Disclosure Letter, Cornerstone has no
Subsidiaries which are not consolidated for accounting purposes. Except for
liabilities and obligations set forth in the Cornerstone SEC Documents or in
Schedule 2.6(c) to the Cornerstone Disclosure Letter, none of Cornerstone, any
Cornerstone Subsidiary or the Cornerstone Non-controlled Subsidiary has any
liabilities or obligations of any nature (whether accrued, absolute, contingent
or otherwise) required by GAAP to be set forth on a consolidated balance sheet
of Cornerstone or in the notes thereto and which, individually or in the
aggregate, would have a Cornerstone Material Adverse Effect.

     2.7 Absence of Certain Changes or Events. Except as disclosed in the
Cornerstone SEC Documents or in Schedule 2.7 to the Cornerstone Disclosure
Letter, since the date of the most recent audited financial statements included
in Cornerstone SEC Documents (the "Cornerstone Financial Statement Date"),
Cornerstone, its Subsidiaries and the Cornerstone Non-controlled Subsidiary have
conducted their business only in the ordinary course (taking into account prior
practices, including the acquisition and disposition of properties and issuance
of securities) and there has not been (a) any material adverse change in the
business, financial condition or results of operations of Cornerstone and its
Subsidiaries taken as a whole (a "Cornerstone Material Adverse Change"), nor has
there been any occurrence or circumstance that with the passage of time would
reasonably be expected to result in a Cornerstone Material Adverse Change, (b)
except for regular quarterly distributions not in excess of $0.31 per share of
Cornerstone Common Stock or Cornerstone OP Unit and except for regular annual
distributions not in excess of $1.155 per share of Cornerstone 7% Preferred
Stock (or, in each case, with respect to the period commencing on the date
hereof and ending on the Closing Date, distributions as necessary to maintain
REIT (as defined herein) status), in each case with customary record and payment
dates, any authorization, declaration, setting aside or payment of any dividend
or other distribution (whether in cash, stock or property) with respect to the
Cornerstone Common Stock, the Cornerstone OP Units or the Cornerstone 7%
Preferred Stock, (c) any split, combination or reclassification of the
Cornerstone Common Stock, the Cornerstone OP Units or the Cornerstone 7%
Preferred Stock or any issuance or the authorization of any issuance of any
other securities in respect of, in lieu of or in substitution for, or giving the
right to acquire by exchange or exercise, shares of stock of Cornerstone or
partnership interests in Cornerstone Partnership or any issuance
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of an ownership interest in, any Cornerstone Subsidiary or the Cornerstone
Non-controlled Subsidiary, (d) any damage, destruction or loss, whether or not
covered by insurance, that has or would have a Cornerstone Material Adverse
Effect, (e) any change in accounting methods, principles or practices by
Cornerstone or any Cornerstone Subsidiary or the Cornerstone Non-controlled
Subsidiary materially affecting its assets, liabilities or business, except
insofar as may have been disclosed in Cornerstone SEC Documents or required by a
change in GAAP, or (f) any amendment of any employment, consulting, severance,
retention or any other agreement between Cornerstone and any officer or director
of Cornerstone.

     2.8 Litigation. Except as disclosed in the Cornerstone SEC Documents or in
Schedule 2.8 to the Cornerstone Disclosure Letter, and other than personal
injury and other routine tort litigation arising from the ordinary course of
operations of Cornerstone, the Cornerstone Subsidiaries and the Cornerstone Non-
controlled Subsidiary (a) which are covered by adequate insurance subject to a
reasonable deductible or retention limit or (b) for which all material costs and
liabilities arising therefrom are reimbursable pursuant to common area
maintenance or similar agreements, there is no suit, action or proceeding
pending (in which service of process has been received by an employee of
Cornerstone, a Cornerstone Subsidiary or a Cornerstone Non-controlled
Subsidiary) or, to the Knowledge of Cornerstone (as hereinafter defined),
threatened in writing against or affecting Cornerstone, or any Cornerstone
Subsidiary or Cornerstone Non-controlled Subsidiary that, individually or in the
aggregate, would reasonably be expected to (i) have a Cornerstone Material
Adverse Effect or (ii) prevent the consummation of any of the transactions
contemplated by this Agreement, nor is there any judgment, decree, injunction,
rule or order of any court or Governmental Entity or arbitrator outstanding
against Cornerstone, any Cornerstone Subsidiary or the Cornerstone
Non-controlled Subsidiary having, or which, insofar as reasonably can be
foreseen, in the future would have, any such effect. Notwithstanding the
foregoing, (y) Schedule 2.8 to the Cornerstone Disclosure Letter sets forth each
and every material uninsured claim, equal employment opportunity claim and claim
relating to sexual harassment and/or discrimination pending or, to the Knowledge
of Cornerstone, threatened as of the date hereof, and (z) no claim has been made
under any directors' and officers' liability insurance policy maintained at any
time by Cornerstone, any of the Cornerstone Subsidiaries or the Cornerstone
Non-controlled Subsidiary.

     2.9 Properties.

     (a) Except as provided in Schedule 2.2 or Schedule 2.9(a) to the
Cornerstone Disclosure Letter, Cornerstone or the Cornerstone Subsidiary or
Cornerstone Non-controlled Subsidiary set forth on Schedule 2.2 to the
Cornerstone Disclosure Letter owns fee simple title to or holds a leasehold
interest in each of the real properties identified in Schedule 2.2 to the
Cornerstone Disclosure Letter (the "Cornerstone Properties"), which are all of
the real estate properties owned by them, in each case (except for the Permitted
Title Exceptions (as defined herein)) free and clear of liens, mortgages or
deeds of trust, claims against title, charges which are liens, security
interests or other encumbrances on title ("Encumbrances"). Schedule 2.2 to the
Cornerstone Disclosure Letter further identifies which of the Cornerstone
Properties are owned in fee simple by Cornerstone or the Cornerstone Subsidiary
or the Cornerstone Non-controlled Subsidiary and which of the Cornerstone
Properties are subject to a ground lease. Except as set forth in Schedule 2.2 to
the Cornerstone Disclosure Letter, no other Person has any ownership interest in
any of the Cornerstone Properties, and any such ownership interest so scheduled
does not materially detract from the value of the Cornerstone Subsidiary's or
Cornerstone Non-controlled Subsidiary's (as the case may be) interest in, or
materially interfere with the present use of, any of the Cornerstone Properties
subject thereto or affected thereby. Except as set forth in Schedule 2.9(a) to
the Cornerstone Disclosure Letter, none of the Cornerstone Properties is subject
to any restriction on the sale or other disposition thereof or on the financing
or release of financing thereon. The Cornerstone Properties are not subject to
any rights of way, written agreements, laws, ordinances and regulations
affecting building use or occupancy, or reservations of an interest in title
(collectively, "Property Restrictions") or Encumbrances, except for the
following (collectively, the "Permitted Title Exceptions") (i) Property
Restrictions and Encumbrances set forth in the Cornerstone Disclosure Letter,
(ii) Property Restrictions imposed or promulgated by law or any governmental
body or authority with respect to real property,

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including zoning regulations, which do not materially adversely affect the
current use of any Cornerstone Property, (iii) Property Restrictions and
Encumbrances disclosed on existing title reports or policies or existing surveys
or subsequently granted by Cornerstone or the Cornerstone Subsidiary or
Cornerstone Non-controlled Subsidiary, which Property Restrictions and
Encumbrances, in any event, do not materially detract from the value of, or
materially interfere with the present use of, any of the Cornerstone Properties
subject thereto or affected thereby and (iv) liens for real estate taxes not yet
due and payable, mechanics', carriers', workmen's, repairmen's liens and other
Encumbrances and Property Restrictions, if any, which, individually or in the
aggregate, do not materially detract from the value of or materially interfere
with the present use of any of the Cornerstone Properties subject thereto or
affected thereby. Schedule 2.9(a) to the Cornerstone Disclosure Letter lists
each of the Cornerstone Properties which are under development as of the date of
this Agreement and describes the status of such development as of the date
hereof.

     (b) Except as provided in Schedule 2.2 or Schedule 2.9(b) to the
Cornerstone Disclosure Letter, valid policies of title insurance or fully-paid
and enforceable commitments therefor have been issued insuring the applicable
Cornerstone Subsidiary's or Cornerstone Non-controlled Subsidiary's (as the case
may be) fee simple title or leasehold estate, as the case may be, to the
Cornerstone Properties owned by it in amounts approximately equal to the
purchase price therefor paid by such Cornerstone Subsidiary or Cornerstone
Non-controlled Subsidiary, subject only to the matters disclosed above and in
the Cornerstone Disclosure Letter. Such policies are, at the date hereof, in
full force and effect. No claim has been made against any such policy.

     (c) Except as provided in Schedule 2.9(c) to the Cornerstone Disclosure
Letter, Cornerstone has no Knowledge (i) that, any certificate, permit or
license from any governmental authority having jurisdiction over any of the
Cornerstone Properties or any agreement, easement or other right which is
necessary to permit the lawful use and operation of the buildings and
improvements on any of the Cornerstone Properties or which is necessary to
permit the lawful use and operation of all driveways, roads and other means of
egress and ingress to and from any of the Cornerstone Properties has not been
obtained and is not in full force and effect, or of any pending threat of
modification or cancellation of any of the same which would have a material
adverse effect on such Cornerstone Property, (ii) of any written notice of any
violation of any federal, state or municipal law, ordinance, order, regulation
or requirement affecting any of the Cornerstone Properties issued by any
governmental authority which would have a material adverse effect on such
Cornerstone Property, (iii) of any structural defects relating to any
Cornerstone Property which would have a material adverse effect on such
Cornerstone Property, (iv) of any Cornerstone Property whose building systems
are not in working order so as to have a material adverse effect on such
Cornerstone Property, or (v) of any physical damage to any Cornerstone Property
which would have a material adverse effect on such Cornerstone Property for
which there is no insurance in effect covering the cost of the restoration.

     (d) None of Cornerstone, any Cornerstone Subsidiary or the Cornerstone
Non-controlled Subsidiary has received any written or published notice to the
effect that (i) any condemnation or rezoning proceedings are pending or
threatened with respect to any of the Cornerstone Properties or (ii) any zoning,
building or similar law, code, ordinance, order or regulation is or will be
violated by the continued maintenance, operation or use of any buildings or
other improvements on any of the Cornerstone Properties or by the continued
maintenance, operation or use of the parking areas which would have a material
adverse effect on such Cornerstone Property. Except as set forth in Schedule
2.9(d) to the Cornerstone Disclosure Letter, all work required to be performed,
payments required to be made and actions required to be taken prior to the date
hereof pursuant to any agreement entered into with a governmental body or
authority in connection with a site approval, zoning reclassification or other
similar action relating to any Cornerstone Properties (e.g., Local Improvement
District, Road Improvement District, Environmental Mitigation) have been
performed, paid or taken, as the case may be, and Cornerstone has no Knowledge
of any planned or proposed work, payments or actions that may be required after
the date hereof pursuant to such agreements, except as set forth in development
or operating budgets for such Cornerstone Properties delivered to EOP and EOP
Partnership prior to the date hereof

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and other than those which would not reasonably be expected to have a material
adverse effect on any of Cornerstone or the Cornerstone Subsidiaries or the
Cornerstone Non-controlled Subsidiary.

     (e) The rent rolls previously provided by Cornerstone to EOP (the
"Cornerstone Rent Roll") list each Cornerstone Space Lease (as defined herein)
in effect as of the dates set forth therein, none of which are earlier than
December 31, 1999. "Cornerstone Space Lease" means each lease or other right of
occupancy affecting or relating to a property in which Cornerstone Partnership
(or an entity in which it directly or indirectly has an interest) is the
landlord, either pursuant to the terms of the lease agreement or as successor to
any prior landlord, but excluding any ground lease. Cornerstone has made
available to EOP true, correct and complete copies of all Cornerstone Space
Leases, including all amendments, modifications, supplements, renewals,
extensions and guarantees related thereto, as of the date hereof. Except for
discrepancies that, either individually or in the aggregate, would not
reasonably be expected to have a Cornerstone Material Adverse Effect, all
information set forth in the Cornerstone Rent Roll is true, correct and complete
as of the date thereof. Except as set forth in a delinquency report made
available to EOP, none of Cornerstone, any Cornerstone Subsidiary or the
Cornerstone Non-controlled Subsidiary, on the one hand, nor, to the knowledge of
Cornerstone or Cornerstone Partnership, any other party, on the other hand, is
in monetary default under any Cornerstone Space Lease, except for such defaults
that would not reasonably be expected to have a Cornerstone Material Adverse
Effect.

     2.10 Environmental Matters.

     (a) "Environmental Law" shall mean all applicable Laws relating to the
protection of human health or safety and natural resources or the environment,
including, without limitation, Laws relating to the use, manufacturing,
generation, recycling, reuse, sale, storage, handling, transport, treatment or
disposal of any Hazardous Materials (including the Comprehensive Environmental
Response, Compensation, and Liability Act, as amended, 42 U.S.C. sec.sec. 9601
et seq. ("CERCLA")). "Hazardous Materials" shall mean substances, wastes or
materials listed, regulated or defined under any Environmental Law, and shall
include "hazardous wastes," "hazardous substances," "hazardous materials,"
petroleum or any fraction thereof, asbestos, lead-paint, urea-formaldehyde, and
polychlorinated biphenyls. "Release" shall have the meaning set forth in Section
101 of CERCLA, without regard to the exclusions set forth therein.

     (b) Except as disclosed in the Cornerstone SEC Documents or in Schedule
2.10(a) to the Cornerstone Disclosure Letter,

          (i) none of Cornerstone, any of the Cornerstone Subsidiaries or the
     Cornerstone Non-controlled Subsidiary or, to Cornerstone's Knowledge, any
     other Person has caused or permitted the presence of any Hazardous
     Materials at, on or under any of the Cornerstone Properties and none of
     Cornerstone, any of the Cornerstone Subsidiaries or the Cornerstone
     Non-controlled Subsidiary has any knowledge of the presence of any
     Hazardous Materials at, on or under any of the Cornerstone Properties, in
     each of the foregoing cases, such that the presence of such Hazardous
     Materials (including the presence of asbestos in any buildings or
     improvements at the Cornerstone Properties) would, individually or in the
     aggregate, reasonably be expected to have a Cornerstone Material Adverse
     Effect;

          (ii) except in accordance with the Environmental Permits (as defined
     herein) there have been no Releases of Hazardous Materials at, on, under or
     from (A) the Cornerstone Properties or (B) any real property previously
     owned, operated or leased by Cornerstone, the Cornerstone Subsidiaries, or
     the Cornerstone Non-controlled Subsidiary (the "Former Cornerstone
     Properties") during the period of such ownership, operation or tenancy, and
     none of Cornerstone, any of the Cornerstone Subsidiaries or the Cornerstone
     Non-controlled Subsidiary has any knowledge of any Releases of Hazardous
     Materials having occurred or presently occurring at, on, under or from the
     Cornerstone Properties or the Former Cornerstone Properties, which would,
     individually or in the aggregate, reasonably be expected to have a
     Cornerstone Material Adverse Effect;

          (iii) Cornerstone, the Cornerstone Subsidiaries and the Cornerstone
     Non-controlled Subsidiary have not failed to comply with all Environmental
     Laws, and none of Cornerstone, any of the Cornerstone Subsidiaries or the
     Cornerstone Non-controlled Subsidiary has any liability under the

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     Environmental Laws, except to the extent that any such failure to comply or
     any such liability, individually or in the aggregate, would not reasonably
     be expected to have a Cornerstone Material Adverse Effect; and

          (iv) Cornerstone, the Cornerstone Subsidiaries and the Cornerstone
     Non-controlled Subsidiary have been duly issued, and currently have and
     will maintain through the Closing Date, all permits, licenses, certificates
     and approvals required under any Environmental Law (collectively, the
     "Environmental Permits") necessary to operate their businesses as currently
     operated except where the failure to obtain and maintain such Environmental
     Permit would not have a material adverse effect on the Cornerstone
     Property. Cornerstone, the Cornerstone Subsidiaries and the Cornerstone
     Non-controlled Subsidiary have timely filed applications for all
     Environmental Permits. All of the Environmental Permits are transferable
     and none require consent, notification or other action to remain in full
     force and effect following consummation of the transactions contemplated
     hereby.

     (c) Cornerstone has previously delivered or made available to EOP complete
copies of all material information, documents and reports, including, without
limitation, environmental investigations and testing or analysis that are in the
possession or control of any of Cornerstone, the Cornerstone Subsidiaries and
the Cornerstone Non-controlled Subsidiary and which relate to compliance with
Environmental Laws by any of them or to the past or current environmental
condition of the Cornerstone Properties.

     2.11 Related Party Transactions. Set forth in Schedule 2.11 to the
Cornerstone Disclosure Letter is a list of all material arrangements, agreements
and contracts entered into by Cornerstone, any Cornerstone Subsidiary and the
Cornerstone Non-controlled Subsidiary with (a) any investment banker or
financial advisor, (b) any person who is an officer, director or Affiliate (as
defined herein) of Cornerstone or any Subsidiary or the Cornerstone
Non-controlled Subsidiary, any relative of any of the foregoing or any entity of
which any of the foregoing is an Affiliate or (c) any person who acquired
Cornerstone Common Stock, Cornerstone OP Units or Cornerstone 7% Preferred Stock
in a private placement, in each case which remain in effect and are not
otherwise disclosed in the SEC Documents. Such documents, copies of all of which
have previously been delivered or made available to EOP, are listed in Schedule
2.11 to the Cornerstone Disclosure Letter. As used in this Agreement, the term
"Affiliate" shall have the same meaning as such term is defined in Rule 405
promulgated under the Securities Act.

     2.12 Employee Benefits. As used herein, the term "Employee Plan" includes
any pension, retirement, savings, disability, medical, dental, health, life,
death benefit, group insurance, profit sharing, deferred compensation, stock
option, bonus, incentive, vacation pay, tuition reimbursement, severance pay, or
other employee benefit plan, trust, agreement, contract, agreement, policy or
commitment (including, without limitation, any pension plan, as defined in
Section 3(2) of the Employee Retirement Income Security Act of 1974, as amended,
and the rules and regulations promulgated thereunder ("ERISA") ("Pension Plan"),
and any welfare plan as defined in Section 3(1) of ERISA ("Welfare Plan")),
whether any of the foregoing is funded, insured or self-funded, written or oral,
(i) sponsored or maintained by Cornerstone, any Cornerstone Subsidiary or the
Cornerstone Non-controlled Subsidiary (each, a "Controlled Group Member") and
covering any Controlled Group Member's active or former employees (or their
beneficiaries), (ii) to which any Controlled Group Member is a party or by which
any Controlled Group Member (or any of the rights, properties or assets thereof)
is bound or (iii) with respect to which any current Controlled Group Member may
otherwise have any material liability (whether or not such Controlled Group
Member still maintains such Employee Plan). Each Employee Plan is listed on
Schedule 2.12 to the Cornerstone Disclosure Letter. Except as disclosed in
Schedule 2.12 to the Cornerstone Disclosure Letter, with respect to the Employee
Plans:

     (a) No Controlled Group Member has any continuing liability under any
Welfare Plan which provides for continuing benefits or coverage for any
participant or any beneficiary of a participant after such participant's
termination of employment, except as may be required by Section 4980B of the
Code or Section 601 (et seq.) of ERISA, or under any applicable state law, and
at the expense of the participant or the beneficiary of the participant.

                                      A-20
<PAGE>   140

     (b) Each Employee Plan complies in all material respects with the
applicable requirements of ERISA and any other applicable law governing such
Employee Plan, and, to the Knowledge of Cornerstone, each Employee Plan has at
all times been properly administered in all material respects in accordance with
all such requirements of law, and in accordance with its terms and the terms of
any applicable collective bargaining agreement to the extent consistent with all
such requirements of law. Each Pension Plan which is intended to be qualified is
qualified under Section 401(a) of the Code, has received a favorable
determination letter from the IRS stating that such Plan meets the requirements
of Section 401(a) of the Code and that the trust associated with such Plan is
tax-exempt under Section 501(a) of the Code and, to the Knowledge of
Cornerstone, no event has occurred which would jeopardize the qualified status
of any such plan or the tax exempt status of any such trust under Sections
401(a) and Section 501(a) of the Code, respectively. No lawsuits, claims (other
than routine claims for benefits) or complaints to, or by, any person or
governmental entity have been filed, are pending or, to the Knowledge of
Cornerstone, threatened with respect to any Employee Plan and, to the Knowledge
of Cornerstone, there is no fact or contemplated event which would be expected
to give rise to any such lawsuit, claim (other than routine claims for benefits)
or complaint with respect to any Pension Plan. Without limiting the foregoing,
the following are true with respect to each Employee Plan:

          (i) all Controlled Group Members have complied in all material
     respects with the reporting and disclosure requirements of ERISA, the Code,
     or both, with respect to each Employee Plan and no Controlled Group Member
     has incurred any material liability in connection with such reporting or
     disclosure;

          (ii) all contributions and payments with respect to Employee Plans
     that are required to be made by a Controlled Group Member with respect to
     periods ending on or before the Closing Date (including periods from the
     first day of the current plan or policy year to the Closing Date) have
     been, or will be, made or accrued before the Closing Date in accordance
     with the appropriate plan document, actuarial report, collective bargaining
     agreements or insurance contracts or arrangements or as otherwise required
     by ERISA or the Code; and

          (iii) with respect to each such Employee Plan, to the extent
     applicable, Cornerstone has delivered to or has made available to EOP true
     and complete copies of (A) plan documents, or any and all other documents
     that establish the existence of the plan, trust, arrangement, contract,
     policy or commitment and all amendments thereto, (B) the most recent
     determination letter, if any, received from the IRS, (C) the three most
     recent Form 5500 Annual Reports (and all schedules and reports relating
     thereto) and actuarial reports and (D) all related trust agreements,
     insurance contract or other funding agreements that implement each such
     Employee Plan.

     (c) With respect to each Employee Plan, to the Knowledge of Cornerstone,
there has not occurred, and no person or entity is contractually bound to enter
into, any "prohibited transaction" within the meaning of Section 4975(c) of the
Code or Section 406 of ERISA, which transaction is not exempt under Section
4975(d) of the Code or Section 408 of ERISA and which could subject Cornerstone
or any Controlled Group Member to material liability.

     (d) No Controlled Group Member has maintained or been obligated to
contribute to any Employee Plan subject to Code Section 412 or Title IV of
ERISA. No Employee Plan subject to Code Section 412 or Title IV of ERISA has
been terminated.

     (e) With respect to each Pension Plan maintained by any Controlled Group
Member, such Plan provides the Plan Sponsor the authority to amend or terminate
the Plan at any time, subject to applicable requirements of ERISA and the Code.

     2.13 Employee Policies. The employee handbooks of Cornerstone, the
Cornerstone Subsidiaries and the Cornerstone Non-controlled Subsidiary currently
in effect are attached as Schedule 2.13 to the Cornerstone Disclosure Letter and
fairly and accurately summarize all material employee policies, vacation
policies and payroll policies.

                                      A-21
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     2.14 Taxes.

     (a) Each of Cornerstone, the Cornerstone Subsidiaries and the Cornerstone
Non-controlled Subsidiary (A) has filed all Tax returns and reports required to
be filed by it (after giving effect to any filing extension properly granted by
a Governmental Entity having authority to do so) and all such returns and
reports are accurate and complete in all material respects, (B) has paid (or
Cornerstone has paid on its behalf) all Taxes (as defined herein) shown on such
returns and reports as required to be paid by it, and (C) has complied in all
material respects with all applicable laws, rules and regulations relating to
the payment and withholding of Taxes (including, without limitation, withholding
of Taxes pursuant to Sections 1441, 1442, 1445, 3121, and 3402 of the Code or
similar provisions under any foreign laws) and has, within the time period
prescribed by law, withheld and paid over to the proper governmental entities
all amounts required to be so withheld and paid over under applicable laws and
regulations, except, with respect to all of the foregoing, where the failure to
file such tax returns and reports or failure to pay such Taxes or failure to
comply with such withholding requirements would not reasonably be expected to
have a Cornerstone Material Adverse Effect. The most recent audited financial
statements contained in the Cornerstone SEC Documents reflect an adequate
reserve for all material Taxes payable by Cornerstone, the Cornerstone
Subsidiaries and the Cornerstone Non-controlled Subsidiary for all taxable
periods and portions thereof through the date of such financial statements.
Since the Cornerstone Financial Statement Date, Cornerstone has incurred no
liability for Taxes under Sections 857(b), 860(c) or 4981 of the Code, including
without limitation any Tax arising from a prohibited transaction described in
Section 857(b)(6) of the Code, and none of Cornerstone, any Cornerstone
Subsidiary or the Cornerstone Non-controlled Subsidiary has incurred any
material liability for Taxes other than in the ordinary course of business. No
event has occurred, and no condition or circumstance exists, which presents a
material risk that any material Tax described in the preceding sentences will be
imposed upon Cornerstone, any Cornerstone Subsidiary, or the Cornerstone
Non-controlled Subsidiary. None of Cornerstone, any Cornerstone Subsidiary or
the Cornerstone Non-controlled Subsidiary is the subject of any audit,
examination, or other proceeding in respect of federal income Taxes, and to
Cornerstone's Knowledge, no audit, examination or other proceeding in respect of
federal income Taxes involving any of Cornerstone, any Cornerstone Subsidiary,
or the Cornerstone Non-controlled Subsidiary is being considered by any Tax
authority. To the Knowledge of Cornerstone, no deficiencies for any Taxes have
been proposed, asserted or assessed against Cornerstone, any Cornerstone
Subsidiary or the Cornerstone Non-controlled Subsidiary, and no requests for
waivers of the time to assess any such Taxes are pending. As used in this
Agreement, "Taxes" shall include all taxes, charges, fees, levies and other
assessments, including, without limitation, income, gross receipts, excise,
property, sales, withholding (including, without limitation, dividend
withholding and withholding required pursuant to Sections 1445 and 1446 of the
Code), social security, occupation, use, service, license, payroll, franchise,
transfer and recording taxes, fees and charges, including estimated taxes,
imposed by the United States or any taxing authority (domestic or foreign),
whether computed on a separate, consolidated, unitary, combined or any other
basis, and any interest, fines, penalties or additional amounts attributable to,
or imposed upon, or with respect to any such taxes, charges, fees, levies or
other assessments.

     (b) Cornerstone (i) for all taxable years for which the Internal Revenue
Service could assert a tax liability, has been subject to taxation as a real
estate investment trust (a "REIT") within the meaning of Section 856 of the Code
and has satisfied all requirements to qualify as a REIT for all such years, (ii)
has operated since December 31, 1999 to the date of this representation, and
intends to continue to operate, in such a manner as to qualify as a REIT for the
taxable year ending on the earlier of December 31, 2000 or the Closing Date and,
if later, for the taxable year of Cornerstone ending on the Closing Date, and
(iii) has not taken or omitted to take any action which would reasonably be
expected to result in a challenge to its status as a REIT and, to Cornerstone's
Knowledge, no such challenge is pending or threatened. Each Cornerstone
Subsidiary which is a partnership, joint venture or limited liability company
(i) has been since its formation and continues to be treated for federal income
tax purposes as a partnership and not as a corporation or an association taxable
as a corporation and (ii) has not since the later of its formation or the
acquisition by Cornerstone of a direct or indirect interest therein, owned any
assets (including, without limitation, securities) that would cause Cornerstone
to violate Section
                                      A-22
<PAGE>   142

856(c)(4) of the Code. Cornerstone Partnership is not a publicly-traded
partnership within the meaning of Section 7704(b) of the Code that is taxable as
a corporation pursuant to Section 7704(a) of the Code. Each Cornerstone
Subsidiary which is a corporation has been since its formation a qualified REIT
subsidiary under Section 856(i) of the Code. Neither Cornerstone nor any
Cornerstone Subsidiary holds any asset (x) the disposition of which would be
subject to rules similar to Section 1374 of the Code as a result of an election
under IRS Notice 88-19 or Temporary Treas. Reg. sec.1.337(d)-5T or (y) which is
subject to a consent filed pursuant to Section 341(f) of the Code and the
regulations thereunder.

     2.15 No Payments to Employees, Officers or Directors. Schedule 2.15 to the
Cornerstone Disclosure Letter contains a true and complete list of all
arrangements, agreements or plans pursuant to which cash and non-cash payments
which will become payable (and the maximum aggregate amount which may be payable
thereunder) to each employee, officer or director of Cornerstone, any
Cornerstone Subsidiary or the Cornerstone Non-controlled Subsidiary as a result
of the Merger or a termination of service subsequent to the consummation of the
Merger. Except as described in Schedule 2.15 to the Cornerstone Disclosure
Letter, or as otherwise provided for in this Agreement, there is no employment
or severance contract, or other agreement requiring payments, cancellation of
indebtedness or other obligation to be made on a change of control or otherwise
as a result of the consummation of any of the transactions contemplated by this
Agreement or as a result of a termination of service subsequent to the
consummation of any of the transactions contemplated by this Agreement, with
respect to any employee, officer or director of Cornerstone, any Cornerstone
Subsidiary or the Cornerstone Non-controlled Subsidiary. There is no agreement
or arrangement with any employee, officer or other service provider under which
Cornerstone, any Cornerstone Subsidiary or any Cornerstone Non-controlled
Subsidiary has agreed to pay any tax that might be owed under Section 4999 of
the Code with respect to payments to such individuals.

     2.16 Broker; Schedule of Fees and Expenses. No broker, investment banker,
financial advisor or other person, other than Lazard Freres & Co. L.L.C. the
fees and expenses of which are described in the engagement letter dated January
28, 2000, between Lazard Freres & Co. L.L.C. and Cornerstone, a true, correct
and complete copy of which has previously been given to EOP, is entitled to any
broker's, finder's, financial advisor's or other similar fee or commission in
connection with the transactions contemplated hereby based upon arrangements
made by or on behalf of Cornerstone or any Cornerstone Subsidiary.

     2.17 Compliance with Laws. None of Cornerstone, any Cornerstone Subsidiary
or the Cornerstone Non-controlled Subsidiary has violated or failed to comply
with any statute, law, ordinance, regulation, rule, judgment, decree or order of
any Governmental Entity applicable to its business, properties or operations,
except to the extent that such violation or failure would not reasonably be
expected to have a Cornerstone Material Adverse Effect.

     2.18 Contracts; Debt Instruments.

     (a) None of Cornerstone, any Cornerstone Subsidiary or the Cornerstone
Non-controlled Subsidiary has received a written notice that it is in violation
of or in default under (nor to the Knowledge of Cornerstone does there exist any
condition which upon the passage of time or the giving of notice or both would
cause such a violation of or default under) any material loan or credit
agreement, note, bond, mortgage, indenture, lease, permit, concession,
franchise, license or any other material contract, agreement, arrangement or
understanding, to which it is a party or by which it or any of its properties or
assets is bound, nor to the Knowledge of Cornerstone does such a violation or
default exist, except to the extent that such violation or default, individually
or in the aggregate, would not reasonably be expected to have a Cornerstone
Material Adverse Effect.

     (b) Except for any of the following expressly identified in Cornerstone SEC
Documents, Schedule 2.18(b) to the Cornerstone Disclosure Letter sets forth a
list of each material loan or credit agreement, note, bond, mortgage, indenture
and any other agreement or instrument pursuant to which any Indebtedness (as
defined herein) of Cornerstone, the Cornerstone Subsidiaries and the Cornerstone
Non-controlled Subsidiary, other than Indebtedness payable to Cornerstone, a
Cornerstone Subsidiary or a Cornerstone Non-controlled Subsidiary, is
outstanding or may be incurred. For purposes of this Section 2.18,
"Indebtedness" shall mean (i) indebtedness for borrowed money, whether secured
or unsecured,
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<PAGE>   143

(ii) obligations under conditional sale or other title retention agreements
relating to property purchased by such person, (iii) capitalized lease
obligations, (iv) obligations under interest rate cap, swap, collar or similar
transaction or currency hedging transactions (valued at the termination value
thereof) and (v) guarantees of any such indebtedness of any other person.

     (c) To the extent not set forth in response to the requirements of Section
2.18(b), Schedule 2.18(c) to the Cornerstone Disclosure Letter sets forth each
interest rate cap, interest rate collar, interest rate swap, currency hedging
transaction, and any other agreement relating to a similar transaction to which
Cornerstone, any Cornerstone Subsidiary or the Cornerstone Non-controlled
Subsidiary is a party or an obligor with respect thereto.

     (d) Except as set forth in Schedule 2.18(d) of the Cornerstone Disclosure
Letter, none of Cornerstone, any Cornerstone Subsidiary or the Cornerstone
Non-controlled Subsidiary is a party to any agreement which would restrict any
of them from prepaying any of their Indebtedness without penalty or premium at
any time or which requires any of them to maintain any amount of Indebtedness
with respect to any of the Cornerstone Properties.

     (e) Except as set forth in Schedule 2.18(e) to the Cornerstone Disclosure
Letter, none of Cornerstone, any Cornerstone Subsidiary or the Cornerstone
Non-controlled Subsidiary is a party to any agreement relating to the management
of any Cornerstone Property by any Person other than Cornerstone, a Cornerstone
Subsidiary or a Cornerstone Non-controlled Subsidiary.

     (f) None of Cornerstone, any Cornerstone Subsidiary or the Cornerstone
Non-controlled Subsidiary is a party to any agreement pursuant to which
Cornerstone, any Cornerstone Subsidiary or the Cornerstone Non-controlled
Subsidiary manages or provides services with respect to any real properties
other than Cornerstone Properties, except for the agreements described in
Schedule 2.18(f) to the Cornerstone Disclosure Letter.

     (g) Cornerstone has delivered to EOP prior to the date of this Agreement a
true and complete capital budget for the year 2000 relating to budgeted capital
improvements and development. Schedule 2.18(g) to the Cornerstone Disclosure
Letter lists all material agreements entered into by Cornerstone, each of the
Cornerstone Subsidiaries and the Cornerstone Non-controlled Subsidiary relating
to the development or construction of, or additions or expansions to, any
Cornerstone Real Properties (or any properties with respect to which Cornerstone
has executed as of the date of this Agreement a purchase agreement or other
similar agreement) which are currently in effect and under which Cornerstone or
any of the Cornerstone Subsidiaries or the Cornerstone Non-controlled Subsidiary
currently has, or expects to incur, an obligation in excess of $250,000 in the
aggregate. True, correct and complete copies of such agreements have previously
been delivered or made available to EOP.

     (h) Schedule 2.18(h) to the Cornerstone Disclosure Letter lists all
agreements entered into by Cornerstone, any Cornerstone Subsidiary or the
Cornerstone Non-controlled Subsidiary providing for the sale of, or option to
sell, any Cornerstone Properties or the purchase of, or option to purchase, by
Cornerstone, any Cornerstone Subsidiary or the Cornerstone Non-controlled
Subsidiary, on the one hand, or the other party thereto, on the other hand, any
real estate which are currently in effect.

     (i) Except as set forth in Schedule 2.18(i) to the Cornerstone Disclosure
Letter, none of Cornerstone, any Cornerstone Subsidiary or the Cornerstone
Non-controlled Subsidiary has any material continuing contractual liability (A)
for indemnification or otherwise under any agreement relating to the sale of
real estate previously owned, whether directly or indirectly, by Cornerstone,
any Cornerstone Subsidiary or Cornerstone Non-controlled Subsidiary or (B) to
pay any additional purchase price for any of the Cornerstone Properties.

     (j) Except as set forth in Schedule 2.18(j) to the Cornerstone Disclosure
Letter, none of Cornerstone, any Cornerstone Subsidiary or the Cornerstone
Non-controlled Subsidiary has entered into or is subject, directly or
indirectly, to any "Tax Protection Agreements." As used herein, a Tax Protection
Agreement is an agreement, oral or written, (A) that has as one of its purposes
to permit a person or entity to take the position that such person or entity
could defer federal taxable income that otherwise
                                      A-24
<PAGE>   144

might have been recognized upon a transfer of property to the Cornerstone
Partnership or any other Cornerstone Subsidiary that is treated as a partnership
for federal income tax purposes, and that (i) prohibits or restricts in any
manner the disposition of any assets of Cornerstone, any Cornerstone Subsidiary
or the Cornerstone Non-controlled Subsidiary, (ii) requires that Cornerstone,
any Cornerstone Subsidiary or the Cornerstone Non-controlled Subsidiary
maintain, or put in place, or replace, indebtedness, whether or not secured by
one or more of the Cornerstone Properties, or (iii) requires that Cornerstone,
any Cornerstone Subsidiary or the Cornerstone Non-controlled Subsidiary offer to
any person or entity at any time the opportunity to guarantee or otherwise
assume, directly or indirectly (including, without limitation, through a
"deficit restoration obligation," indemnification agreement or other similar
arrangement), the risk of loss for federal income tax purposes for indebtedness
or other liabilities of Cornerstone, any Cornerstone Subsidiary or the
Cornerstone Non-controlled Subsidiary, (B) that specifies or relates to a method
of taking into account book-tax disparities under Section 704(c) of the Code
with respect to one or more assets of Cornerstone or a Cornerstone Subsidiary,
or (C) that requires a particular method for allocating one or more liabilities
of Cornerstone or any Cornerstone Subsidiary under Section 752 of the Code. None
of Cornerstone, any Cornerstone Subsidiary or the Cornerstone Non-controlled
Subsidiary is in violation of or in default under any Tax Protection Agreement.

     (k) Except as set forth in Schedule 2.18(k) to the Cornerstone Disclosure
Letter and for the Confidentiality Agreement, dated January 13, 2000 between
Cornerstone and EOP (the "Confidentiality Agreement") and other confidentiality
agreements entered into in the ordinary course of business, neither Cornerstone
nor any Cornerstone Subsidiary or the Cornerstone Non-controlled Subsidiary is a
party to any confidentiality, standstill, lock-up or voting agreement.

     2.19 Opinion of Financial Advisor. Cornerstone has received the written
opinion of Lazard Freres & Co. L.L.C. Cornerstone's financial advisor, to the
effect that the proposed consideration to be received by the holders of
Cornerstone Common Stock is fair to such holders from a financial point of view.

     2.20 State Takeover Statutes. Cornerstone has taken all action necessary to
exempt the transactions contemplated by this Agreement between EOP and
Cornerstone and its Affiliates from the operation of any "fair price,"
"moratorium," "control share acquisition" or any other anti-takeover statute or
similar statute enacted under the laws of the State of Nevada and the State of
Delaware or federal laws of the United States or similar statute or regulation
(a "Takeover Statute").

     2.21 Investment Company Act of 1940. None of Cornerstone, any Cornerstone
Subsidiary or the Cornerstone Non-controlled Subsidiary is, or at the Effective
Time will be, required to be registered under the Investment Company Act of
1940, as amended (the "1940 Act").

     2.22 Definition of "Knowledge of Cornerstone". As used in this Agreement,
the phrase "Knowledge of Cornerstone" (or words of similar import) means the
actual knowledge of those individuals identified in Schedule 2.22 to the
Cornerstone Disclosure Letter.

     2.23 Required Stockholder Approvals and Partner Approvals. The affirmative
vote of the holders of at least a majority of the outstanding Cornerstone Common
Stock and the holders of at least a majority of the outstanding Cornerstone 7%
Preferred Stock are the only votes of the holders of any class or series of
Cornerstone capital stock necessary or required under this Agreement or under
applicable law to approve the Merger and this Agreement. The approval of
Cornerstone and the affirmative vote of at least a majority of all Cornerstone
limited partner interests entitled to be cast, voting in accordance with the
Cornerstone Partnership Agreement, is the only vote of the holders of any class
or series of Cornerstone Partnership's partnership interests necessary or
required under this Agreement or under applicable law to approve the Merger, the
withdrawal of Cornerstone as general partner and the Partnership Merger.

                                      A-25
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                                   ARTICLE 3

           REPRESENTATIONS AND WARRANTIES OF EOP AND EOP PARTNERSHIP

     Except as set forth in the EOP SEC Documents (as defined herein) or in the
letter of even date herewith signed by the President or an Executive Vice
President of EOP and delivered to Cornerstone prior to the execution hereof (the
"EOP Disclosure Letter"), EOP and EOP Partnership represent and warrant to
Cornerstone and Cornerstone Partnership as follows:

     3.1 Organization, Standing and Power of EOP. EOP is a real estate
investment trust duly organized, validly existing and in good standing under the
laws of Maryland and has all requisite power and authority to own, operate,
lease and encumber its properties and carry on its business as now being
conducted. EOP is duly qualified or licensed to do business as a foreign trust
and is in good standing in each jurisdiction in which the nature of its business
or the ownership or leasing of its properties makes such qualification or
licensing necessary, other than in such jurisdictions where the failure to be so
qualified or licensed, individually or in the aggregate, would not reasonably be
expected to have a material adverse effect on the business, properties, assets,
financial condition or results of operations of EOP and the Subsidiaries of EOP
(collectively, "EOP Subsidiaries"), taken as a whole (an "EOP Material Adverse
Effect"). EOP has delivered to Cornerstone complete and correct copies of the
EOP Declaration of Trust (as the same is proposed to be modified by each of the
Proposed EOP Charter Amendments (as defined herein)) and the EOP Bylaws, as
amended or supplemented to the date of this Agreement.

     3.2 EOP Subsidiaries.

     (a) Schedule 3.2(a) to the EOP Disclosure Letter sets forth (i) each EOP
Subsidiary and each entity in which EOP holds non-voting equity securities (but
no voting equity securities) (collectively, the "EOP Non-controlled
Subsidiaries"), (ii) the ownership interest therein of EOP, (iii) if not wholly
owned by EOP, the identity and ownership interest of each of the other owners of
such EOP Subsidiary, (iv) each office property and other commercial property
owned by such Subsidiary, and (v) if not wholly owned by such Subsidiary, the
identity and ownership interest of each of the other owners of such property.

     (b) Except as set forth in Schedule 3.2(b) to the EOP Disclosure Letter,
(i) all the outstanding shares of capital stock of each EOP Subsidiary and each
EOP Non-controlled Subsidiary that is a corporation have been duly authorized,
validly issued and are (A) fully paid and nonassessable and not subject to
preemptive rights, (B) owned by EOP or by another EOP Subsidiary and (C) owned
free and clear of all Liens and (ii) all equity interests in each EOP Subsidiary
that is a partnership, joint venture, limited liability company or trust which
are owned by EOP, by another EOP Subsidiary or by EOP and another EOP Subsidiary
are owned free and clear of all Liens other than pledges, if any, contained in
organizational documents of such EOP Subsidiary and given to secure performance
thereunder. Each EOP Subsidiary that is a corporation is duly incorporated,
validly existing and in good standing under the laws of its jurisdiction of
incorporation and has the requisite corporate power and authority to own,
operate, lease and encumber its properties and carry on its business as now
being conducted, and each EOP Subsidiary that is a partnership, limited
liability company or trust is duly organized, validly existing and in good
standing under the laws of its jurisdiction of organization and has the
requisite power and authority to own, operate, lease and encumber its properties
and carry on its business as now being conducted. Each EOP Subsidiary is duly
qualified or licensed to do business and is in good standing in each
jurisdiction in which the nature of its business or the ownership or leasing of
its properties makes such qualification or licensing necessary, other than in
such jurisdictions where the failure to be so qualified or licensed,
individually or in the aggregate, would not reasonably be expected to have an
EOP Material Adverse Effect. Complete and correct copies of the Articles of
Incorporation, Bylaws, organization documents and partnership, joint venture and
operating agreements of each EOP Subsidiary, as amended to the date of this
Agreement, have been previously delivered or made available to Cornerstone. No
effective amendment has been made to the EOP Partnership Agreement since January
31, 2000.

                                      A-26
<PAGE>   146

     3.3 Capital Structure.

     (a) The authorized shares of beneficial interest of EOP consist of
750,000,000 EOP Common Shares, 248,812,828 of which were issued and outstanding
as of January 31, 2000, and 100,000,000 preferred shares of beneficial interest,
18,562,900 of which were issued or outstanding as of January 31, 2000
(collectively, the "EOP Preferred Shares").

     (b) Set forth in Schedule 3.3(b) to the EOP Disclosure Letter is a true and
complete list of the following: (i) each qualified or nonqualified option to
purchase EOP's shares of beneficial interest granted under the Employee Share
Option and Restricted Share Plan or any other formal or informal arrangement
(collectively, the "EOP Options"); and (ii) all other warrants or other rights
to acquire EOP's shares of beneficial interest, all share appreciation rights,
phantom shares, dividend equivalents, performance units and performance shares
which are outstanding on the date of this Agreement. Schedule 3.3(b) to the EOP
Disclosure Letter sets forth the EOP Options granted to EOP's Chief Executive
Officer and four other most highly compensated officers, the date of each grant,
the status of each EOP Option as qualified or nonqualified under Section 422 of
the Code, the number of EOP Common Shares subject to each EOP Option, the number
and type of EOP's Common Shares subject to EOP Options that are currently
exercisable, the exercise price per share, and the number and type of such
shares subject to share appreciation rights. On the date of this Agreement,
except as set forth in this Section 3.3 or in Schedule 3.3(b) to the EOP
Disclosure Letter, no shares of beneficial interest of EOP were outstanding or
reserved for issuance (except for EOP Common Shares reserved for issuance upon
redemption of EOP OP Units).

     (c) All outstanding shares of beneficial interest of EOP are duly
authorized, validly issued, fully paid and nonassessable and not subject to
preemptive rights. There are no bonds, debentures, notes or other indebtedness
of EOP having the right to vote (or convertible into, or exchangeable for,
securities having the right to vote) on any matters on which shareholders of EOP
may vote.

     (d) Except (i) as set forth in this Section 3.3 or in Schedule 3.3(d) to
the EOP Disclosure Letter and (ii) EOP OP Units, which may be redeemed for EOP
Common Shares, as of the date of this Agreement, there are no outstanding
securities, options, warrants, calls, rights, commitments, agreements,
arrangements or undertakings of any kind to which EOP or any EOP Subsidiary is a
party or by which such entity is bound, obligating EOP or any EOP Subsidiary to
issue, deliver or sell, or cause to be issued, delivered or sold, additional
shares of beneficial interest, voting securities or other ownership interests of
EOP or any EOP Subsidiary or obligating EOP or any EOP Subsidiary to issue,
grant, extend or enter into any such security, option, warrant, call, right,
commitment, agreement, arrangement or undertaking (other than to EOP or an EOP
Subsidiary).

     (e) As of January 31, 2000, 282,476,889 EOP OP Units, 8,000,000 8.98%
Series A Preferred Units of Limited Partnership Interest, 6,000,000 5.25% Series
B Preferred Units of Limited Partnership Interest, and 4,562,900 8 5/8% Series C
Preferred Units of Limited Partnership Interest (collectively, the "EOP
Preferred Units") are validly issued and outstanding, fully paid and
nonassessable and not subject to preemptive rights, of which 248,812,828 EOP OP
Units and all of the EOP Preferred Units are owned by EOP and EOP Subsidiaries.
Schedule 3.3(e) to the EOP Disclosure Schedule sets forth the name of each
holder of EOP OP Units and the number of EOP OP Units owned by each such holder
as of the date of this Agreement. The EOP OP Units and EOP Preferred Units are
subject to no restrictions except as set forth in the EOP Partnership Agreement.
EOP Partnership has not issued or granted and is not a party to any outstanding
commitments of any kind relating to, or any presently effective agreements or
understandings with respect to, interests in EOP Partnership, whether issued or
unissued, or securities convertible or exchangeable into interests in EOP
Partnership.

     (f) All dividends on EOP Common Shares and all distributions on EOP OP
Units and EOP Preferred Units, which have been declared prior to the date of
this Agreement have been paid in full, except that the dividends payable on EOP
Common Shares (along with the corresponding distributions payable on EOP OP
Units) which were declared on February 1, 2000 and are payable on April 14, 2000
have not yet been paid.

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<PAGE>   147

     (g) The EOP Common Shares and the EOP Preferred Shares to be issued by EOP,
and the EOP OP Units to be issued by the EOP Partnership pursuant to this
Agreement have been duly authorized for issuance, and upon issuance will be duly
and validly issued, fully paid and nonassessable.

     3.4 Other Interests. Except for interests in the EOP Subsidiaries and
certain other entities as set forth in Schedule 3.2(a), 3.2(b) or 3.4(a) to the
EOP Disclosure Letter (the "EOP Other Interests"), neither EOP nor any of its
Subsidiaries owns directly or indirectly any interest or investment (whether
equity or debt) in any corporation, partnership, joint venture, business, trust
or other entity (other than investments in short-term investment securities).
With respect to such other interests, EOP or EOP Partnership is a partner or
shareholder in good standing, and owns such interests free and clear of all
Liens other than pledges, if any, contained in organizational documents of such
EOP Other Interests and given to secure performance. Neither EOP nor any of the
EOP Subsidiaries is in breach of any provision of any agreement, document or
contract governing its rights in or to the interests owned or held by it, all of
which agreements, documents and contracts are (a) listed in Schedule 3.4(b) to
the EOP Disclosure Letter (or disclosed in the EOP SEC Documents (as defined
herein)), (b) unmodified except as described therein and (c) in full force and
effect. To the Knowledge of EOP (as defined herein), the other parties to any
such agreement, document or contract which is of a material nature are not in
breach of any of their respective obligations under such agreements, documents
or contracts.

     3.5 Authority; Noncontravention; Consents.

     (a) EOP has the requisite power and authority to enter into this Agreement
and, subject to the requisite shareholder approval of the Merger (the "EOP
Shareholder Approvals" and, together with the Cornerstone Stockholder Approvals,
the "Shareholder Approvals"), to consummate the transactions contemplated by
this Agreement to which EOP is a party. The execution and delivery of this
Agreement by EOP and the consummation by EOP of the transactions contemplated by
this Agreement to which EOP is a party have been duly authorized by all
necessary action on the part of EOP, except for and subject to the EOP
Shareholder Approvals and the requisite approval, if any is required, of the
partners of EOP Partnership. This Agreement has been duly executed and delivered
by EOP and constitutes a valid and binding obligation of EOP, enforceable
against EOP in accordance with and subject to its terms, subject to applicable
bankruptcy, insolvency, moratorium or other similar laws relating to creditors'
rights and general principles of equity.

     (b) EOP Partnership has the requisite partnership power and, subject to the
requisite partner approval of the Partnership Merger (if any), authority to
enter into this Agreement and to consummate the transactions contemplated by
this Agreement to which EOP Partnership is a party. The execution and delivery
of this Agreement by EOP Partnership and the consummation by EOP Partnership of
the transactions contemplated by this Agreement to which EOP Partnership is a
party have been duly authorized by all necessary action on the part of EOP
Partnership, except for and subject to the EOP Partnership Approvals. This
Agreement has been duly executed and delivered by EOP Partnership and
constitutes a valid and binding obligation of EOP Partnership, enforceable
against EOP Partnership in accordance with and subject to its terms, subject to
applicable bankruptcy, insolvency, moratorium or other similar laws relating to
creditors' rights and general principles of equity.

     (c) Except as set forth in Schedule 3.5(c)(1) to the EOP Disclosure Letter,
the execution and delivery of this Agreement by EOP and EOP Partnership do not,
and the consummation of the transactions contemplated by this Agreement to which
EOP or EOP Partnership is a party and compliance by EOP or EOP Partnership with
the provisions of this Agreement will not, conflict with, or result in any
violation of or default (with or without notice or lapse of time, or both)
under, or give rise to a right of termination, cancellation or acceleration of
any material obligation or to loss of a material benefit under, or result in the
creation of any Lien upon any of the properties or assets of EOP or any EOP
Subsidiary under, (i) the EOP Declaration of Trust or the EOP Bylaws or the
comparable charter or organizational documents or partnership, operating or
similar agreement (as the case may be) of any other EOP Subsidiary, each as
amended or supplemented to the date of this Agreement, (ii) any loan or credit
agreement, note, bond, mortgage, indenture, reciprocal easement agreement, lease
or other agreement,

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instrument, permit, concession, franchise or license applicable to EOP or any
EOP Subsidiary or their respective properties or assets or (iii) subject to the
governmental filings and other matters referred to in the following sentence,
any Laws applicable to EOP or any EOP Subsidiary or their respective properties
or assets, other than, in the case of clause (ii) or (iii), any such conflicts,
violations, defaults, rights, loss or Liens that individually or in the
aggregate would not (x) reasonably be expected to have an EOP Material Adverse
Effect or (y) prevent the consummation of the transactions contemplated by this
Agreement. No consent, approval, order or authorization of, or registration,
declaration or filing with, any Governmental Entity is required by or with
respect to EOP or any EOP Subsidiary in connection with the execution and
delivery of this Agreement or the consummation by EOP of any of the transactions
contemplated by this Agreement, except for (i) the filing with the SEC of (x)
the Form S-4 (as defined herein) and (y) such reports under Section 13 (a) of
the Exchange Act as may be required in connection with this Agreement and the
transactions contemplated by this Agreement, (ii) the acceptance for record of
the Articles of Merger by the Department, (iii) the filing of the Certificate of
Merger with the Office of the Secretary of State of the State of Delaware, (iv)
such filings as may be required in connection with the payment of any transfer
and gains taxes and (v) such other consents, approvals, orders, authorizations,
registrations, declarations and filings (A) as are set forth in Schedule
3.5(c)(2) to the EOP Disclosure Letter or (B) as may be required under (x)
federal, state or local environmental laws or (y) the "blue sky" laws of various
states, to the extent applicable, or (C) which, if not obtained or made, would
not prevent or delay in any material respect the consummation of any of the
transactions contemplated by this Agreement or otherwise prevent EOP from
performing its obligations under this Agreement in any material respect or
reasonably be expected to have, individually or in the aggregate, an EOP
Material Adverse Effect.

     3.6 SEC Documents; Financial Statements; Undisclosed Liabilities. EOP and
EOP Partnership have filed all required reports, schedules, forms, statements
and other documents with the SEC since July 8, 1997 and November 19, 1997,
respectively, through the date hereof (the "EOP SEC Documents"). Schedule 3.6(a)
to the EOP Disclosure Letter contains a complete list of all EOP SEC Documents
filed by EOP and EOP Partnership with the SEC under the Exchange Act on or prior
to the date of this Agreement. All of the EOP SEC Documents (other than
preliminary material), as of their respective filing dates, complied in all
material respects with all applicable requirements of the Securities Act and the
Exchange Act and, in each case, the rules and regulations promulgated thereunder
applicable to such EOP SEC Documents. None of the EOP SEC Documents at the time
of filing contained any untrue statement of a material fact or omitted to state
any material fact required to be stated therein or necessary in order to make
the statements therein, in light of the circumstances under which they were
made, not misleading, except to the extent such statements have been modified or
superseded by later EOP SEC Documents filed and publicly available prior to the
date of this Agreement. The consolidated financial statements of EOP and the EOP
Subsidiaries included in the EOP SEC Documents complied as to form in all
material respects with applicable accounting requirements and the published
rules and regulations of the SEC with respect thereto, have been prepared in
accordance with GAAP (except, in the case of unaudited statements, as permitted
by the applicable rules and regulations of the SEC) applied on a consistent
basis during the periods involved (except as may be indicated in the notes
thereto) and fairly presented in all material respects, in accordance with the
applicable requirements of GAAP and the applicable rules and regulations of the
SEC, the consolidated financial position of EOP and the EOP Subsidiaries, taken
as a whole, as of the dates thereof and the consolidated results of operations
and cash flows for the periods then ended (subject, in the case of unaudited
statements, to normal year-end audit adjustments). Except for liabilities and
obligations set forth in the EOP SEC Documents or in Schedule 3.6(b) to the EOP
Disclosure Letter, neither EOP nor any EOP Subsidiary has any liabilities or
obligations of any nature (whether accrued, absolute, contingent or otherwise)
required by GAAP to be set forth on a consolidated balance sheet of EOP or in
the notes thereto and which, individually or in the aggregate, would reasonably
be expected to have an EOP Material Adverse Effect.

     3.7 Absence of Certain Changes or Events. Except as disclosed in the EOP
SEC Documents or in Schedule 3.7 to the EOP Disclosure Letter, since the date of
the most recent audited financial statements included in the EOP SEC Documents
(the "EOP Financial Statement Date"), EOP and the EOP
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<PAGE>   149

Subsidiaries have conducted their business only in the ordinary course (taking
into account prior practices, including the acquisition of properties and
issuance of securities) and there has not been (a) any material adverse change
in the business, financial condition or results of operations of EOP and the EOP
Subsidiaries taken as a whole (a "EOP Material Adverse Change"), nor has there
been any occurrence or circumstance that with the passage of time would
reasonably be expected to result in an EOP Material Adverse Change, (b) except
for regular quarterly distributions not in excess of $0.42 per EOP Common Share
or EOP OP Unit or the stated distribution rate for each EOP Preferred Share or
EOP Preferred Unit, subject to rounding adjustments as necessary and with
customary record and payment dates, any authorization, declaration, setting
aside or payment of any dividend or other distribution (whether in cash, shares
or property) with respect to EOP Common Shares, EOP OP Units, EOP Preferred
Shares or EOP Preferred Units, (c) any split, combination or reclassification of
any of EOP's shares of beneficial interest, (d) any damage, destruction or loss,
whether or not covered by insurance, that has or would reasonably be expected to
have an EOP Material Adverse Effect or (e) any change made prior to the date of
this Agreement in accounting methods, principles or practices by EOP or any EOP
Subsidiary materially affecting its assets, liabilities or business, except
insofar as may have been disclosed in the EOP SEC Documents or required by a
change in GAAP.

     3.8 Litigation. Except as disclosed in the EOP SEC Documents or in Schedule
3.8 to the EOP Disclosure Letter, and other than personal injury and other
routine tort litigation arising from the ordinary course of operations of EOP
and the EOP Subsidiaries (a) which are covered by adequate insurance subject to
a reasonable deductible or retention limit or (b) for which all material costs
and liabilities arising therefrom are reimbursable pursuant to common area
maintenance or similar agreements, there is no suit, action or proceeding
pending (in which service of process has been received by an employee of EOP or
an EOP Subsidiary) or, to the Knowledge of EOP (as defined herein), threatened
in writing against or affecting EOP or any EOP Subsidiary that, individually or
in the aggregate, would reasonably be expected to (i) have an EOP Material
Adverse Effect or (ii) prevent the consummation of any of the transactions
contemplated by this Agreement, nor is there any judgment, decree, injunction,
rule or order of any Governmental Entity or arbitrator outstanding against EOP
or any EOP Subsidiary having, or which, insofar as reasonably can be foreseen,
in the future would have, any such effect.

     3.9 Properties.

     (a) Except as set forth in Schedule 3.9(a) to the EOP Disclosure Letter,
EOP or one of the EOP Subsidiaries owns fee simple title to each of the real
properties listed in the EOP SEC Filings as owned by it (the "EOP Properties"),
except where the failure to own such title would not have an EOP Material
Adverse Effect.

     (b) The EOP Properties are not subject to any Encumbrances or Property
Restrictions which reasonably could be expected to cause an EOP Material Adverse
Effect.

     (c) Valid policies of title insurance or fully-paid and enforceable
commitments therefor have been issued insuring EOP's or the applicable EOP
Subsidiary's fee simple title or leasehold estate, as the case may be, to the
EOP Properties in amounts which are, except in the case of San Felipe Plaza, at
least equal to the purchase price thereof paid by EOP or the applicable EOP
Subsidiaries therefor, except where the failure to obtain such title insurance
would not reasonably be expected to have an EOP Material Adverse Effect.

     (d) EOP has no Knowledge (i) that it has failed to obtain a certificate,
permit or license from any governmental authority having jurisdiction over any
of the EOP Properties where such failure would reasonably be expected to have an
EOP Material Adverse Effect or of any pending threat of modification or
cancellation of any of the same which would reasonably be expected to have an
EOP Material Adverse Effect, (ii) of any written notice of any violation of any
federal, state or municipal law, ordinance, order, rule, regulation or
requirement affecting any of the EOP Properties issued by any governmental
authorities which would reasonably be expected to have an EOP Material Adverse
Effect or (iii) of any structural defects relating to EOP Properties, EOP
Properties whose building systems are not in working order, physical damage to
any EOP Property for which there is no insurance in effect covering the cost of
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<PAGE>   150

restoration, any current renovation or uninsured restoration, except such
structural defects, building systems not in working order, physical damage,
renovation and restoration which, in the aggregate, would not reasonably be
expected to have an EOP Material Adverse Effect.

     (e) Except as set forth in Schedule 3.9(e) to the EOP Disclosure Letter,
neither EOP nor any of the EOP Subsidiaries has received any written or
published notice to the effect that (i) any condemnation or rezoning proceedings
are pending or threatened with respect to any of the EOP Properties or (ii) any
zoning, building or similar law, code, ordinance, order or regulation is or will
be violated by the continued maintenance, operation or use of any buildings or
other improvements on any of the EOP Properties or by the continued maintenance,
operation or use of the parking areas, other than such notices which, in the
aggregate, would not reasonably be expected to have an EOP Material Adverse
Effect.

     (f) Except as set forth on Schedule 3.9(f) to the EOP Disclosure Letter,
all work to be performed, payments to be made and actions to be taken by EOP or
the EOP Subsidiaries prior to the date hereof pursuant to any agreement entered
into with a governmental body or authority in connection with a site approval,
zoning reclassification or similar action relating to any EOP Properties (e.g.,
Local Improvement District, Road Improvement District, Environmental
Mitigation), has been performed, paid or taken, as the case may be, except where
the failure to do so would, in the aggregate, not reasonably be expected to have
an EOP Material Adverse Effect, and EOP has no Knowledge of any planned or
proposed work, payments or actions that may be required after the date hereof
pursuant to such agreements.

     (g) The rent roll previously provided by EOP to Cornerstone (the "EOP Rent
Roll") lists each EOP Space Lease (as defined herein) in effect as of the
respective dates indicated in the EOP Rent Roll. "EOP Space Lease" means each
lease or other right of occupancy affecting or relating to a property in which
EOP Partnership (or an entity in which it directly or indirectly has an
interest) is the landlord, either pursuant to the terms of the lease agreement
or as successor to any prior landlord, but excluding any ground lease. EOP has
made available to Cornerstone true, correct and complete copies of all EOP Space
Leases, including all amendments, modifications, supplements, renewals,
extensions and guarantees related thereto, as of the date hereof. Except for
discrepancies that, either individually or in the aggregate, would not
reasonably be expected to have an EOP Material Adverse Effect, all information
set forth in the EOP Rent Roll is true, correct, and complete as of the date
thereof. Except as set forth in a delinquency report made available to
Cornerstone, neither EOP nor any EOP Subsidiary, on the one hand, nor, to the
Knowledge of EOP or EOP Partnership, any other party, on the other hand, is in
monetary default under any EOP Space Lease, except for such defaults that would
not reasonably be expected to have an EOP Material Adverse Effect.

     3.10 Environmental Matters.

     Except as disclosed in the EOP SEC Documents,

     (i) none of EOP, any of the EOP Subsidiaries or, to EOP's Knowledge, any
other Person has caused or permitted the presence of any Hazardous Materials at,
on or under any of the EOP Properties, such that the presence of such Hazardous
Materials (including the presence of asbestos in any buildings or improvements
at the EOP Properties) would, individually or in the aggregate, reasonably be
expected to have an EOP Material Adverse Effect;

     (ii) except in accordance with the Environmental Permits, there have been
no Releases of Hazardous Materials at, on, under or from (A) the EOP Properties,
or (B) any real property formerly owned, operated or leased by EOP or the EOP
Subsidiaries during the period of such ownership, operation or tenancy, which
would, individually or in the aggregate, reasonably be expected to have an EOP
Material Adverse Effect;

     (iii) EOP and the EOP Subsidiaries have not failed to comply in any
material respect with all Environmental Laws, and neither EOP nor any of the EOP
Subsidiaries has any liability under the Environmental Laws, except to the
extent such failure to comply or any such liability, individually or in the
aggregate, would not reasonably be expected to have an EOP Material Adverse
Effect; and

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<PAGE>   151

     (iv) EOP and the EOP Subsidiaries have been duly issued, and currently have
and will maintain through the Closing Date, all Environmental Permits except
where the failure to obtain and maintain such Environmental Permits would not
have a material adverse effect on the EOP Property necessary to operate their
businesses as currently operated. EOP and the EOP Subsidiaries have timely filed
applications for all Environmental Permits.

     3.11 Taxes.

     (a) Each of EOP and the EOP Subsidiaries (i) has filed all Tax returns and
reports required to be filed by it (after giving effect to any filing extension
properly granted by a Governmental Entity having authority to do so), and all
such returns and reports are accurate and complete in all material respects,
(ii) has paid (or EOP has paid on its behalf) all Taxes shown on such returns
and reports as required to be paid by it and (iii) has complied in all material
respects with all applicable laws, rules and regulations relating to the payment
and withholding of Taxes (including, without limitation, withholding of Taxes
pursuant to Sections 1441, 1442, 1445, 3121, and 3402 of the Code or similar
provisions under any foreign laws) and has, within the time period prescribed by
law, withheld and paid over to the proper governmental entities all amounts
required to be so withheld and paid over under applicable laws and regulations,
except, with respect to all of the foregoing, where the failure to file such tax
returns or reports or failure to pay such Taxes or failure to comply with such
requirements would not reasonably be expected to have an EOP Material Adverse
Effect. The most recent audited financial statements contained in the EOP SEC
Documents reflect an adequate reserve for all material Taxes payable by EOP and
the EOP Subsidiaries for all taxable periods and portions thereof through the
date of such financial statements. Since the EOP Financial Statement Date, EOP
has incurred no liability for Taxes under Sections 857(b), 860(c) or 4981 of the
Code, including without limitation any Tax arising from a prohibited transaction
described in Section 857(b)(6) of the Code, and neither EOP nor any EOP
Subsidiary has incurred any material liability for Taxes other than in the
ordinary course of business. No event has occurred, and no condition or
circumstance exists, which presents a material risk that any material Tax
described in the preceding sentence will be imposed upon EOP or any EOP
Subsidiary. Neither EOP nor any EOP Subsidiary is the subject of any audit,
examination, or other proceeding in respect of federal income Taxes, and to
EOP's Knowledge, no audit, examination or other proceeding in respect of federal
income Taxes involving any of EOP or any EOP Subsidiary is being considered by
any Tax authority. To the Knowledge of EOP, no deficiencies for any Taxes have
been proposed, asserted or assessed against EOP or any of the EOP Subsidiaries,
and no requests for waivers of the time to assess any such Taxes are pending.

     (b) EOP (i) for all taxable years commencing with 1997, which is the first
year of EOP's existence for federal income tax purposes, through December 31,
1999, has been subject to taxation as a REIT within the meaning of Section 856
of the Code and has qualified as a REIT for all such years, (ii) has operated
since December 31, 1999 to the date of this representation, and intends to
continue to operate, in such a manner as to qualify as a REIT for the taxable
year that includes the Closing Date and (iii) has not taken or omitted to take
any action which would reasonably be expected to result in a challenge to its
status as a REIT and, to EOP's Knowledge, no such challenge is pending or
threatened. BeaMetFed, Inc. ("BeaMet") (i) for all taxable years commencing with
1997 through December 31, 1999 has been subject to taxation as a REIT within the
meaning of Section 856 of the Code and has qualified as a REIT for all such
years, (ii) has operated since December 31, 1999 to the date of this
representation, and intends to continue to operate, in such a manner as to
qualify as a REIT for the taxable year that includes the Closing Date and (iii)
has not taken or omitted to take any action which would reasonably be expected
to result in a challenge to its status as a REIT and, to EOP's Knowledge, no
such challenge is pending or threatened. Each EOP Subsidiary which is a
partnership, joint venture or limited liability company (i) has been treated
since its formation and continues to be treated for federal income tax purposes
as a partnership and not as a corporation or as an association taxable as a
corporation and (ii) has not since the later of its formation or the acquisition
by EOP of a direct or indirect interest therein, owned any assets (including,
without limitation, securities) that would cause EOP to violate Section
856(c)(4) of the Code. EOP Partnership is not a publicly-traded partnership
within the meaning of Section 7704(b) of the Code that is taxable as a
corporation pursuant to Section 7704(a) of the Code. Each EOP Subsidiary

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which is a corporation has been since its formation a qualified REIT subsidiary
under Section 856(i) of the Code. Except as set forth in Schedule 3.11 to the
EOP Disclosure Letter, neither EOP nor any EOP Subsidiary holds any asset (x)
the disposition of which would be subject to rules similar to Section 1374 of
the Code as a result of an election under IRS Notice 88-19 or Temporary Treas.
Reg. sec.1.337(d)-5T or (y) which is subject to a consent filed pursuant to
Section 341(f) of the Code and the regulations thereunder.

     (c) To EOP's knowledge, as of the date hereof, EOP is a
"domestically-controlled REIT" within the meaning of Section 897(h)(4)(B) of the
Code.

     3.12 Brokers; Schedule of Fees and Expenses. No broker, investment banker,
financial advisor or other person, other than J.P. Morgan & Co. Incorporated,
the fees and expenses of which will be paid by EOP and are described in the
engagement letters between J.P. Morgan & Co. Incorporated and EOP, a true,
correct and complete copy of which has previously been given to Cornerstone, is
entitled to any broker's, finder's, financial advisor's or other similar fee or
commission in connection with the transactions contemplated hereby based upon
arrangements made by or on behalf of EOP or any EOP Subsidiary.

     3.13 Compliance with Laws. Neither EOP nor any of the EOP Subsidiaries has
violated or failed to comply with any statute, law, ordinance, regulation, rule,
judgment, decree or order of any Governmental Entity applicable to its business,
properties or operations, except to the extent that such violation or failure
would not reasonably be expected to have an EOP Material Adverse Effect.

     3.14 Contracts; Debt Instruments. Neither EOP nor any EOP Subsidiary has
received a written notice that EOP or any EOP Subsidiary is in violation of or
in default under (nor to the Knowledge of EOP does there exist any condition
which upon the passage of time or the giving of notice or both would cause such
a violation of or default under) any material loan or credit agreement, note,
bond, mortgage, indenture, lease, permit, concession, franchise, license or any
other material contract, agreement, arrangement or understanding, to which it is
a party or by which it or any of its properties or assets is bound, nor to the
Knowledge of EOP does such a violation or default exist, except to the extent
such violation or default, individually or in the aggregate, would not
reasonably be expected to have an EOP Material Adverse Effect, except as set
forth in the EOP SEC Documents or in Schedule 3.14 to the EOP Disclosure Letter.

     3.15 Opinion of Financial Advisor. EOP has received the opinion of J.P.
Morgan & Co. Incorporated, EOP's financial advisor, to the effect that the
consideration to be paid by EOP in connection with the Merger is fair, from a
financial point of view, to EOP.

     3.16 State Takeover Statutes. EOP has taken all action necessary to exempt
transactions between EOP and Cornerstone and its Affiliates from the operation
of Takeover Statutes.

     3.17 Investment Company Act of 1940. Neither EOP nor any of the EOP
Subsidiaries is, or at the Effective Time will be, required to be registered
under the 1940 Act.

     3.18 Definition of "Knowledge of EOP". As used in this Agreement, the
phrase "Knowledge of EOP" (or words of similar import) means the actual
knowledge of those individuals identified in Schedule 3.19 to the EOP Disclosure
Letter.

     3.19 Required Shareholder Approvals and Partner Approvals. The affirmative
vote of the holders of not less than a majority of all votes entitled to be cast
by holders of EOP Common Shares, and the affirmative vote of the partners of EOP
Partnership holding at least a majority of the outstanding EOP Partnership
interests (including partnership interests held by EOP), are the only votes of
the holders of any class or series of EOP capital shares necessary or required
under this Agreement or under applicable law to approve the Merger and this
Agreement. The approval of the Partnership Merger by EOP is the only vote
necessary or required under this Agreement or under applicable law to approve
the Partnership Merger and this Agreement.

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                                   ARTICLE 4

                                   COVENANTS

     4.1 Conduct of Cornerstone's and Cornerstone Partnership's Business Pending
Merger. During the period from the date of this Agreement to the Effective
Times, except as consented to in writing by EOP or as expressly provided for in
this Agreement, Cornerstone and Cornerstone Partnership shall, and shall cause
(or, in the case of Cornerstone Subsidiaries and the Cornerstone Non-controlled
Subsidiary that Cornerstone or Cornerstone Partnership do not control, shall use
commercially reasonable efforts to cause) each of the Cornerstone Subsidiaries
and Cornerstone Non-controlled Subsidiary to:

     (a) conduct its business only in the usual, regular and ordinary course and
in substantially the same manner as heretofore conducted;

     (b) preserve intact its business organizations and goodwill and use
commercially reasonable efforts to keep available the services of its officers
and employees;

     (c) confer on a regular basis with one or more representatives of EOP to
report operational matters of materiality and, subject to Section 4.3, any
proposals to engage in material transactions;

     (d) promptly notify EOP of any material emergency or other material change
in the condition (financial or otherwise), business, properties, assets,
liabilities or the normal course of its businesses or in the operation of its
properties, or of any material governmental complaints, investigations or
hearings (or communications indicating that the same may be contemplated);

     (e) promptly deliver to EOP true and correct copies of any report,
statement or schedule filed with the SEC subsequent to the date of this
Agreement;

     (f) maintain its books and records in accordance with GAAP consistently
applied and not change in any material manner any of its methods, principles or
practices of accounting in effect at the Cornerstone Financial Statement Date,
except as may be required by the SEC, applicable law or GAAP;

     (g) duly and timely file all reports, tax returns and other documents
required to be filed with federal, state, local and other authorities, subject
to extensions permitted by law, provided Cornerstone notifies EOP that it is
availing itself of such extensions and provided such extensions do not adversely
affect Cornerstone's status as a qualified REIT under the Code;

     (h) not make or rescind any express or deemed election relative to Taxes
(unless required by law or necessary to preserve Cornerstone's status as a REIT
or the status of any Cornerstone Subsidiary as a partnership for federal income
tax purposes or as a qualified REIT subsidiary under Section 856(i) of the Code,
as the case may be);

     (i) not (A) acquire, enter into any option to acquire, or exercise an
option or other right or election or enter into any other commitment or
contractual obligation (each, a "Commitment") for the acquisition of any real
property or, except as permitted in a budget approved in writing by EOP, other
transaction (other than Commitments referred to in Schedule 4.1(i) to the
Cornerstone Disclosure Letter) involving in excess of $100,000, encumber assets
or commence construction of, or enter into any Commitment to develop or
construct other real estate projects, except in the ordinary course of its
office property business, including leasing activities pursuant to EOP-approved
guidelines (B) incur or enter into any Commitment to incur additional
indebtedness (secured or unsecured) except for working capital under its
revolving line(s) of credit and Commitments for indebtedness described on
Schedule 4.1(i) to the Cornerstone Disclosure Letter or (C) modify, amend or
terminate, or enter into any Commitment to modify, amend or terminate, any
indebtedness (secured or unsecured) in existence as of the date hereof;

     (j) not amend the Cornerstone Articles or the Cornerstone Bylaws, or the
articles or certificate of incorporation, bylaws, code of regulations,
partnership agreement, operating agreement or joint venture agreement or
comparable charter or organization document of any Cornerstone Subsidiary or
Cornerstone Non-controlled Subsidiary;

                                      A-34
<PAGE>   154

     (k) make no change in the number of shares of capital stock or units of
limited partnership interest issued and outstanding, other than pursuant to (i)
the exercise of options disclosed in Schedule 2.3 to the Cornerstone Disclosure
Letter, (ii) the conversion of the Cornerstone 7% Preferred Stock or the
Cornerstone Convertible Promissory Note if required by their terms, (iii) the
redemption of Cornerstone OP Units under existing contracts described on
Schedule 2.18, (iv) the redemption of Cornerstone OP Units pursuant to Section
1.1 of this Agreement, (v) the redemption of Cornerstone OP Units under the
Cornerstone Partnership Agreement solely for shares of Cornerstone Common Stock,
or (vi) Cornerstone OP Units issuable upon the exercise of the First and Howard
option;

     (l) grant no options or other right or commitment relating to its shares of
capital stock or units of limited partnership interest or any security
convertible into its shares of capital stock or units of limited partnership
interest, or any security the value of which is measured by shares of beneficial
interest, or any security subordinated to the claim of its general creditors
and, other than or pursuant to Section 5.8(c) or (d) of this Agreement, not
amend or waive any rights under any of the Cornerstone Stock Options or
Cornerstone Stock Rights;

     (m) except as provided in Section 5.10 and in connection with the use of
Cornerstone Common Stock to pay the exercise price or tax withholding in
connection with equity-based employee benefit plans by the participants therein,
not (i) authorize, declare, set aside or pay any dividend or make any other
distribution or payment with respect to any Cornerstone Common Stock,
Cornerstone 7% Preferred Stock or Cornerstone OP Units or (ii) directly or
indirectly redeem, purchase or otherwise acquire any shares of capital stock or
units of partnership interest or any option, warrant or right to acquire, or
security convertible into, shares of capital stock or units of partnership
interest of Cornerstone, except for (A) deemed transfers of Cornerstone excess
shares required under Article 8 of the Cornerstone Articles in order to preserve
the status of Cornerstone as a REIT under the Code or Article 9 of the
Cornerstone Articles, and (B) redemptions of Cornerstone OP Units, whether or
not outstanding on the date of this Agreement, under the Cornerstone Partnership
Agreement in which solely Cornerstone Common Stock is utilized;

     (n) not sell, lease, mortgage, subject to Lien or otherwise dispose of any
of the Cornerstone Properties, except in connection with a transaction that is
permitted by Section 4.1(i), that is made in the ordinary course of business and
is the subject of a binding contract in existence on the date of this Agreement
and disclosed in Schedule 2.18 to the Cornerstone Disclosure Letter or in
connection with a transaction that is permitted by the leasing guidelines set
forth on Schedule 4.1(n) to the Cornerstone Disclosure Letter;

     (o) not sell, lease, mortgage, subject to Lien or otherwise dispose of any
of its personal property or intangible property, except in the ordinary course
of business and is not material, individually or in the aggregate;

     (p) not make any loans, advances or capital contributions to, or
investments in, any other Person, other than loans, advances and capital
contributions to Cornerstone Subsidiaries or the Cornerstone Non-controlled
Subsidiary in existence on the date hereof and ordinary course expense advances
to employees and except in connection with a transaction permitted by Section
4.1(i), and not enter into any new, or amend or supplement any existing,
contract, lease or other agreement with the Cornerstone Non-controlled
Subsidiary;

     (q) not pay, discharge or satisfy any claims, liabilities or obligations
(absolute, accrued, asserted or unasserted, contingent or otherwise), other than
the payment, discharge or satisfaction, in the ordinary course of business
consistent with past practice or in accordance with their terms, of liabilities
reflected or reserved against in, or contemplated by, the most recent
consolidated financial statements (or the notes thereto) furnished to EOP or
incurred in the ordinary course of business consistent with past practice;

     (r) not guarantee the indebtedness of another Person, enter into any "keep
well" or other agreement to maintain any financial statement condition of
another Person or enter into any arrangement having the economic effect of any
of the foregoing;

                                      A-35
<PAGE>   155

     (s) except as disclosed in Schedule 4.1(s) of the Cornerstone Disclosure
Letter, not enter into any Commitment with any officer, director or Affiliate of
Cornerstone or any of the Cornerstone Subsidiaries or the Cornerstone
Non-controlled Subsidiary or any material Commitment with any consultant;

     (t) except as disclosed in Schedule 4.1(t) of the Cornerstone Disclosure
Letter, not increase any compensation or enter into or amend any employment
agreement described in Schedule 2.18 to the Cornerstone Disclosure Letter with
any of its officers, directors or employees earning more than $100,000 per
annum, other than as required by any contract or Plan or in accordance with
waivers by employees of benefits under such agreements;

     (u) not adopt any new employee benefit plan or amend any existing plans or
rights;

     (v) not settle any stockholder derivative or class action claims arising
out of or in connection with any of the transactions contemplated by this
Agreement;

     (w) not change the ownership of any of its Subsidiaries or the Cornerstone
Non-controlled Subsidiary, except changes which arise as a result of the
acquisition of Cornerstone OP Units in exchange for Cornerstone Common Stock
pursuant to exercise of the Cornerstone OP Unit redemption right under Section
8.6 of the Cornerstone Partnership Agreement;

     (x) not accept a promissory note in payment of the exercise price payable
under any option to purchase shares of Cornerstone Common Stock;

     (y) not enter into any Tax Protection Agreement;

     (z) not settle or compromise any material federal, state, local or foreign
tax liability; and

     (aa) not authorize, recommend, propose or announce an intention to do any
of the foregoing prohibited actions, or enter into any contract, agreement,
commitment or arrangement to do any of the foregoing prohibited actions.

     4.2 Conduct of EOP's and EOP Partnership's Business Pending Merger.  During
the period from the date of this Agreement to the Effective Times, except as
consented to in writing by Cornerstone or as expressly provided for in this
Agreement, EOP and EOP Partnership shall, and shall cause (or, in the case of
Cornerstone Subsidiaries that Cornerstone or Cornerstone Partnership do not
control, shall use commercially reasonable efforts to cause) each of the EOP
Subsidiaries to:

     (a) preserve intact its business organizations and goodwill and use
commercially reasonable efforts to keep available the services of its officers
and employees;

     (b) confer on a regular basis with one or more representatives of
Cornerstone to report operational matters of materiality which would reasonably
be expected to have an EOP Material Adverse Effect;

     (c) promptly notify Cornerstone of any material emergency or other material
change in the condition (financial or otherwise), business, properties, assets,
liabilities, prospects or the normal course of its businesses or in the
operation of its properties, or of any material governmental complaints,
investigations or hearings (or communications indicating that the same may be
contemplated);

     (d) promptly deliver to Cornerstone true and correct copies of any report,
statement or schedule filed with the SEC subsequent to the date of this
Agreement;

     (e) maintain its books and records in accordance with GAAP consistently
applied and not change in any material manner any of its methods, principles or
practices of accounting in effect at the EOP Financial Statement Date, except as
may be required by the SEC, applicable law or GAAP;

     (f) duly and timely file all reports, tax returns and other documents
required to be filed with federal, state, local and other authorities, subject
to extensions permitted by law, provided such extensions do not adversely affect
EOP's status as a qualified REIT under the Code;

     (g) not make or rescind any express or deemed election relative to Taxes
(unless required by law or necessary to preserve EOP's status as a REIT or the
status of any EOP Subsidiary as a partnership for
                                      A-36
<PAGE>   156

federal income tax purposes or as a qualified REIT subsidiary under Section
856(i) of the Code, as the case may be);

     (h) not amend the EOP Declaration of Trust, the EOP Bylaws or the EOP
Partnership Agreement (except for the Proposed EOP Charter Relating to Voting
Requirements, the Proposed EOP Charter Amendment Relating to Domestically
Controlled REIT Status (as defined herein), the proposed amendment to Section
7.4 of the EOP Partnership Agreement, the amendments to the EOP Partnership
Agreement described in Section 5.4(c), the filing of Articles Supplementary to
elect to be subject to the provisions of Section 3-804(c) of the Maryland
General Corporation Law and to the extent necessary to reflect the admission of
additional limited partners and other amendments in connection therewith that
can be made by EOP without a vote of limited partners and that will not,
individually or in the aggregate, materially adversely affect the rights or
obligations of holders of EOP OP Units); as used herein, (i) "Proposed EOP
Charter Amendment Relating to Certain Voting Requirements" means the proposed
amendment to EOP's Declaration of Trust, substantially the form attached hereto
as Exhibit D, which has been approved by the Board of Trustees of EOP and is
proposed to be submitted to a vote of the shareholders of EOP, (ii) "Proposed
EOP Charter Amendment Relating to Domestically Controlled REIT Status" means the
proposed amendment to EOP's Declaration of Trust, substantially the form
attached hereto as Exhibit E, which has been approved by the Board of Trustees
of EOP and is proposed to be submitted to a vote of the shareholders of EOP at
the EOP Shareholders Meeting (as defined below) and (iii) "Proposed EOP Charter
Amendments" means, collectively, the Proposed EOP Charter Relating to Voting
Requirements and the Proposed EOP Charter Amendment Relating to Domestically
Controlled REIT Status;

     (i) except as provided in Section 5.10 hereof and in connection with the
use of EOP Common Shares to pay the exercise price or tax withholding in
connection with equity-based employee benefit plans by the participants therein,
not (i) authorize, declare, set aside or pay any dividend or make any other
distribution or payment with respect to any EOP Common Shares or EOP OP Units or
(ii) directly or indirectly redeem, purchase or otherwise acquire any shares of
capital stock, membership interests or units of partnership interest or any
option, warrant or right to acquire, or security convertible into, shares of
capital stock, membership interests, or units of partnership interest of EOP,
except for (A) redemptions of EOP Common Shares required under Section 7.3.6 of
the EOP Declaration of Trust in order to preserve the status of EOP as a REIT
under the Code, (B) redemptions of EOP OP Units, whether or not outstanding on
the date of this Agreement, under the EOP Partnership Agreement in which EOP
Common Shares are utilized, and (C) repurchases of EOP Common Shares pursuant to
its publicly announced share repurchase program;

     (j) not (A) enter into or agree to effect any merger, acquisition,
consolidation, reorganization or other business combination with any third party
in which EOP is not the surviving party thereto or (B) enter into or agree to
effect any merger, acquisition, exchange offer or other business combination
with a third party in which EOP is the surviving party that would result in the
issuance of equity securities representing in excess of 20% of the outstanding
EOP Common Shares on the date any such business combination is entered into or
agreed to unless, in either such case, such business combination is approved by
Cornerstone, which approval shall not be unreasonably withheld or delayed, or
the business combination agreement provides that the required vote of EOP
shareholders for approval of such business combination is no less than the
affirmative vote of holders of EOP Common Shares representing more than 50% of
the sum of (x) the number of EOP Common Shares outstanding at the time of such
approval plus (y) 50,000,000; and

     (k) not authorize, recommend, propose or announce an intention to do any of
the foregoing prohibited actions, or enter into any contract, agreement,
commitment or arrangement to do any of the foregoing prohibited actions.

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<PAGE>   157

     4.3 No Solicitation.

     (a) Prior to the Effective Time of the Merger, Cornerstone agrees, for
itself and in its capacity as the sole general partner of the Cornerstone
Partnership, that:

          (i) none of it, Cornerstone Partnership, any Cornerstone Subsidiary or
     the Cornerstone Non-controlled Subsidiary shall invite, initiate, solicit
     or encourage, directly or indirectly, any inquiries, proposals, discussions
     or negotiations or the making or implementation of any proposal or offer
     (including, without limitation, any proposal or offer to its stockholders)
     with respect to a merger, acquisition, tender offer, exchange offer,
     transaction resulting in the issuance of securities representing 10% or
     more of the outstanding equity securities of Cornerstone or Cornerstone
     Partnership, consolidation, business combination, recapitalization,
     liquidation, dissolution, share exchange, business combination, sale,
     lease, exchange, mortgage, pledge, transfer or other disposition of 10% or
     more of the assets or equity securities (including, without limitation,
     partnership interests and units) of Cornerstone or Cornerstone Partnership,
     other than the Mergers (any such proposal or offer being hereinafter
     referred to as an "Acquisition Proposal"), or engage in any discussions or
     negotiations with or provide any confidential or non-public information or
     data to, any person relating to, or that may reasonably be expected to lead
     to, an Acquisition Proposal, or enter into any letter of intent, agreement
     in principle or agreement relating to an Acquisition Proposal, or propose
     publicly to agree to do any of the foregoing, or otherwise facilitate any
     effort or attempt to make or implement an Acquisition Proposal;

          (ii) none of it, Cornerstone Partnership, any Cornerstone Subsidiary
     or the Cornerstone Non-controlled Subsidiary will permit any officer,
     director, employee, affiliate, agent, investment banker, financial advisor,
     attorney, accountant, broker, finder, consultant or other agent or
     representative of Cornerstone to engage in any of the activities described
     in Section 4.3(a);

          (iii) it, Cornerstone Partnership and the Cornerstone Subsidiaries and
     Cornerstone Non-controlled Subsidiary will immediately cease and cause to
     be terminated any existing activities, discussions or negotiations with any
     parties conducted heretofore with respect to any of the foregoing and will
     take the necessary steps to inform the individuals or entities referred to
     in Section 4.3(b) of the obligations undertaken in this Section 4.3; and

          (iv) it will notify EOP promptly (but in any event within 24 hours) if
     Cornerstone, Cornerstone Partnership, any Cornerstone Subsidiary, the
     Cornerstone Non-controlled Subsidiary or any individual or entity referred
     to in Section 4.3(b) receives any such inquiries or proposals, or any
     requests for such information, or if any such negotiations or discussions
     are sought to be initiated or continued with it, and include in such notice
     the identity of the Person making such inquiry, proposal or request, the
     material terms of such inquiry, proposal or request and, if in writing,
     shall promptly deliver to EOP a copy of such inquiry, proposal or request
     along with all other related documentation and correspondence;

     (b) Notwithstanding Section 4.3(a), the Board of Directors of Cornerstone
(including with respect to Cornerstone's capacity as the sole general partner of
Cornerstone Partnership) shall not be prohibited from furnishing information to
or entering into discussions or negotiations with, any person or entity that
makes an unsolicited bona fide written Acquisition Proposal, if, and only to the
extent that (i) a majority of the Board of Directors of Cornerstone determines
in good faith, after consultation with its outside counsel, that such action is
required for the Board of Directors of Cornerstone to comply with its fiduciary
duties to stockholders imposed by applicable law, (ii) prior to furnishing such
information to, or entering into discussions or negotiations with, such person
or entity, Cornerstone provides written notice to EOP to the effect that it is
furnishing information to, or entering into discussions with such person or
entity and (iii) Cornerstone enters into a confidentiality agreement with such
Person on terms in the aggregate not more favorable to such Person than the
terms of the Confidentiality Agreement.

     (c) Notwithstanding anything to the contrary set forth in Section 4.3(a) or
4.3(b), in the event that an Acquisition Proposal constitutes a Superior
Acquisition Proposal (as defined herein), nothing contained in this Section 4.3
shall prohibit the Board of Directors of Cornerstone from withdrawing,
modifying,

                                      A-38
<PAGE>   158

amending or qualifying its recommendation of this Agreement and the Merger as
required under Section 5.1(d) hereof and recommending such Superior Acquisition
Proposal to its stockholders: (i) if but only if, Cornerstone: (A) complies
fully with this Section 4.3 and (B) provides EOP with at least three (3)
business days' prior written notice of its intent to withdraw, modify, amend or
qualify its recommendation of this Merger Agreement and the Merger, (ii) if, in
the event that during such three (3) business days EOP makes a counter proposal
to such Superior Acquisition Proposal (any such counter proposal being referred
to in this Agreement as the "EOP Counter Proposal"), Cornerstone's Board of
Directors in good faith, taking into account the advice of its outside financial
advisors of nationally recognized reputation, determines (A) that the EOP
Counter Proposal is not at least as favorable to Cornerstone's stockholders as
the Superior Acquisition Proposal, from a financial point of view, and (B) the
EOP Counter Proposal is not at least as favorable generally to Cornerstone's
stockholders (taking into account all financial and strategic considerations and
other relevant factors, including relevant legal, financial, regulatory and
other aspects of such proposals, and the conditions, prospects and time required
for completion of such proposal), and (iii) Cornerstone shall have terminated
this Agreement in accordance with Section 7.1(h).

     (d) For all purposes of this Agreement, "Superior Acquisition Proposal"
means a bona fide written proposal made by a third party to acquire, directly or
indirectly, Cornerstone and/or Cornerstone Partnership pursuant to a tender or
exchange offer, merger, share exchange, consolidation or sale of all or
substantially all of the assets of Cornerstone, Cornerstone Partnership, and the
Cornerstone Subsidiaries or otherwise (i) on terms which a majority of the Board
of Directors of Cornerstone determines in good faith, (A) taking into account
the advice of Cornerstone's financial advisors of nationally recognized
reputation, are superior, from a financial point of view, to Cornerstone's
stockholders to those provided for in the Merger and (B) to be more favorable
generally to Cornerstone's stockholders (taking into account all financial and
strategic considerations and other relevant factors, including relevant legal,
financial, regulatory and other aspects of such proposals, and the conditions,
prospects and time required for completion of such proposal), (ii) for which
financing, to the extent required, is then fully committed and capable of being
obtained and (iii) which the Board of Directors of Cornerstone determines in
good faith is reasonably capable of being consummated.

     (e) Any disclosure that the Board of Directors of Cornerstone may be
compelled to make with respect to the receipt of an Acquisition Proposal in
order to comply with its duties to shareholders imposed by applicable law or
Rule 14d-9 or 14e-2 of the Exchange Act will not constitute a violation of this
Section 4.3.

     (f) Nothing in this Section 4.3 shall (i) permit Cornerstone to terminate
this Agreement (except as expressly provided in Article 7) or (ii) affect any
other obligations of Cornerstone under this Agreement.

     4.4 Affiliates. Prior to the Effective Time of the Merger, Cornerstone
shall cause to be prepared and delivered to EOP a list (reasonably satisfactory
to counsel for EOP) identifying all persons who, at the time of the Cornerstone
Stockholders Meeting and the EOP Shareholders Meeting, may be deemed to be
"affiliates" of Cornerstone or Cornerstone Partnership as that term is used in
paragraphs (c) and (d) of Rule 145 under the Securities Act (the "Rule 145
Affiliates"). Cornerstone shall use its commercially reasonable efforts to cause
each person who is identified as a Rule 145 Affiliate in such list to deliver to
EOP on or prior to the Effective Time a written agreement, in the form
previously approved by the parties hereto, that such Rule 145 Affiliate will not
sell, pledge, transfer or otherwise dispose of any EOP Common Shares and EOP OP
Units issued to such Rule 145 Affiliate pursuant to the Merger, except pursuant
to an effective registration statement under the Securities Act or in compliance
with paragraph (d) of Rule 145 or as otherwise permitted by the Securities Act.
EOP shall be entitled to place legends as specified in such written agreements
on the certificates representing any EOP Common Shares to be received pursuant
to the terms of this Agreement by such Rule 145 Affiliates who have executed
such agreements and to issue appropriate stop transfer instructions to the
transfer agent for the EOP Common Shares and EOP OP Units issued to such Rule
145 Affiliates, consistent with the terms of such agreements. EOP and EOP OP
shall timely file the reports required to be filed by it under the Exchange Act
and the rules and regulations adopted by the SEC thereunder, and it will take
such further action as

                                      A-39
<PAGE>   159

any Rule 145 Affiliate of Cornerstone or EOP may reasonably request, all to the
extent required from time to time to enable such Rule 145 Affiliate to sell
shares of beneficial interest of EOP received by such Rule 145 Affiliate in the
Merger without registration under the Securities Act pursuant to (i) Rule
145(d)(1) under the Securities Act, as such rule may be amended from to time, or
(ii) any successor rule or regulation hereafter adopted by the SEC.

     4.5 Other Actions. Each of Cornerstone and Cornerstone Partnership, on the
one hand, and EOP and EOP Partnership, on the other hand, shall not take, and
shall use commercially reasonable efforts to cause their respective Subsidiaries
not to take, any action that would result in (i) any of the representations and
warranties of such party (without giving effect to any "knowledge"
qualification) set forth in this Agreement that are qualified as to materiality
becoming untrue, (ii) any of such representations and warranties (without giving
effect to any "knowledge" qualification) that are not so qualified becoming
untrue in any material respect or (iii) except as contemplated by Section 4.3,
any of the conditions to the Merger set forth in Article 6 not being satisfied.

                                   ARTICLE 5

                              ADDITIONAL COVENANTS

     5.1 Preparation of the Form S-4 and the Proxy Statement; Cornerstone
Stockholders Meeting, Cornerstone Unitholders Consent Solicitation and EOP
Shareholders Meeting.

     (a) As promptly as practicable after execution of this Agreement, (i) each
of Cornerstone and EOP shall prepare and file with the SEC (with appropriate
requests for confidential treatment, unless the parties hereto otherwise agree)
under the Exchange Act, one or more joint proxy statements/prospectuses, forms
of proxies and information statements (such joint proxy
statement(s)/prospectus(es) and information statements together with any
amendments to supplements thereto, the "Joint Proxy Statement") relating to the
stockholder meeting of Cornerstone and the shareholder meeting of EOP, the vote
of the stockholders of Cornerstone with respect to this Agreement and the
shareholders of EOP with respect to the Merger and the Proposed EOP Charter
Amendment Relating to Domestically Controlled REIT Status, and the consent, if
any, of partners of Cornerstone Partnership and EOP Partnership in connection
with any required Partner Approvals and (ii) in connection with the clearance by
the SEC of the Joint Proxy Statement, EOP and Cornerstone, if applicable, shall
prepare and file with the SEC under the Securities Act one or more registration
statements on Form S-4 (such registration statements, together with any
amendments or supplements thereto, the "Form S-4"), in which the Joint Proxy
Statement will be included, as one or more prospectuses in connection with the
registration under the Securities Act of the EOP Common Shares and EOP OP Units
to be distributed to the holders of Cornerstone Common Stock and Cornerstone OP
Units in the Mergers. The respective parties will cause the Proxy Statement and
the Form S-4 to comply as to form in all material respects with the applicable
provisions of the Securities Act, the Exchange Act and the rules and regulations
thereunder. Each of Cornerstone, Cornerstone Partnership, EOP and EOP
Partnership shall furnish all information about itself and its business and
operations and all necessary financial information to the other as the other may
reasonably request in connection with the preparation of the Joint Proxy
Statement and the Form S-4. EOP and Cornerstone, if applicable, shall use its
commercially reasonable efforts, and Cornerstone will cooperate with it, to have
the Form S-4 declared effective by the SEC as promptly as practicable (including
clearing the Proxy Statement with the SEC. Each of Cornerstone and Cornerstone
Partnership, on the one hand, and EOP and EOP Partnership, on the other hand,
agree promptly to correct any information provided by it for use in the Joint
Proxy Statement and the Form S-4 if and to the extent that such information
shall have become false or misleading in any material respect, and each of the
parties hereto further agrees to take all steps necessary to amend or supplement
the Joint Proxy Statement and the Form S-4 and to cause the Joint Proxy
Statement and the Form S-4 as amended or supplemented to be filed with the SEC
and to be disseminated to their respective stockholders and shareholders and
partners, in each case as and to the extent required by applicable federal and
state securities laws. Each of Cornerstone, Cornerstone Partnership, EOP and EOP
Partnership agrees that the information provided by it for inclusion in the
Joint Proxy Statement or the Form S-4 and each amendment or supplement thereto,
at the time of

                                      A-40
<PAGE>   160

mailing thereof and at the time of the respective meetings of stockholders and
shareholders of Cornerstone and EOP and at the time of the respective taking of
consents, if any, of partners of Cornerstone Partnership and EOP Partnership,
will not include an untrue statement of a material fact or omit to state a
material fact required to be stated therein or necessary to make the statements
therein, in light of the circumstances under which they were made, not
misleading. EOP will advise and deliver copies (if any) to Cornerstone, promptly
after it receives notice thereof, of any request by the SEC for amendment of the
Joint Proxy Statement or the Form S-4 or comments thereon and responses thereto
or requests by the SEC for additional information (regardless of whether such
requests relate to EOP or EOP Partnership, on the one hand, and Cornerstone or
Cornerstone Partnership, on the other hand), and EOP shall promptly notify
Cornerstone, and Cornerstone shall promptly notify EOP, if applicable, of (i)
the time when the Form S-4 has become effective, (ii) the filing of any
supplement or amendment thereto, (iii) the issuance of any stop order, and (iv)
the suspension of the qualification and registration of the EOP Common Shares
and EOP OP Units issuable in connection with the Mergers.

     (b) Each of Cornerstone, Cornerstone Partnership, EOP and EOP Partnership
shall use its commercially reasonable efforts to timely mail the joint proxy
statement/prospectus contained in the Form S-4 to its stockholders or
shareholders. It shall be a condition to the mailing of the joint proxy
statement/prospectus that (i) EOP and EOP Partnership shall have received a
"comfort" letter from PricewaterhouseCoopers LLP, independent public accountants
for Cornerstone and Cornerstone Partnership, of the kind contemplated by the
Statement of Auditing Standards with respect to Letters to Underwriters
promulgated by the American Institute of Certified Public Accountants (the
"AICPA Statement"), dated as of the date on which the Form S-4 shall become
effective and as of the Effective Time, addressed to EOP and EOP Partnership, in
form and substance reasonably satisfactory to EOP and EOP Partnership,
concerning the procedures undertaken by PricewaterhouseCoopers LLP with respect
to the financial statements and information of Cornerstone and Cornerstone
Partnership and their subsidiaries and the Cornerstone Non-controlled Subsidiary
contained in the Form S-4 and the other matters contemplated by the AICPA
Statement and otherwise customary in scope and substance for letters delivered
by independent public accountants in connection with transactions such as those
contemplated by this Agreement and (ii) Cornerstone shall have received a
"comfort" letter from Ernst & Young LLP, independent public accountants for EOP
and EOP Partnership, of the kind contemplated by the AICPA Statement, dated as
of the date on which the Form S-4 shall become effective and as of the Effective
Time, addressed to Cornerstone and Cornerstone Partnership, in form and
substance reasonably satisfactory to Cornerstone, concerning the procedures
undertaken by Ernst & Young LLP with respect to the financial statements and
information of EOP, EOP Partnership and their subsidiaries contained in the Form
S-4 and the other matters contemplated by the AICPA Statement and otherwise
customary in scope and substance for letters delivered by independent public
accountants in connection with transactions such as those contemplated by this
Agreement.

     (c) EOP will duly call and give notice of and, as soon as practicable
following the date of this Agreement (but in no event sooner than 20 business
days following the date the Joint Proxy Statement is mailed to the shareholders
of EOP), convene and hold a meeting of its shareholders (the "EOP Shareholders
Meeting") for the purpose of obtaining the EOP Shareholder Approvals. EOP shall,
through its Board of Trustees, recommend to its shareholders approval of this
Agreement, the Merger and the transactions contemplated by this Agreement.

     (d) Cornerstone will duly call and give notice of and, as soon as
practicable following the date of this Agreement (but in no event sooner than 20
business days following the date the Joint Proxy Statement is mailed to the
stockholders of Cornerstone), convene and hold a meeting of its stockholders
(the "Cornerstone Stockholders Meeting") for the purpose of obtaining the
Cornerstone Stockholder Approvals. Cornerstone shall, through its Board of
Directors, recommend to its stockholders approval of this Agreement, the Merger
and the transactions contemplated by this Agreement and include such
recommendation in the Proxy Statement; provided, however, that prior to the
Cornerstone Stockholders Meeting, such recommendation may be withdrawn,
modified, amended or qualified if and only to the extent permitted by Section
4.3(c) hereof.

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     (e) EOP and Cornerstone shall use their commercially reasonable efforts to
convene their respective shareholder and stockholder meetings on the same day,
which day, subject to the provisions of Sections 5.1(c), 5.1(d) and 5.3, shall
be a day not later than 60 days after the date the Joint Proxy Statement is
mailed.

     (f) If on the date for the EOP Shareholders Meeting and Cornerstone
Stockholders Meeting established pursuant to Section 5.1(e) of this Agreement,
either EOP or Cornerstone has not received duly executed proxies for a
sufficient number of votes to approve the Merger, then both parties shall
recommend the adjournment of their respective shareholders and stockholders
meetings until one or more dates not later than the date 10 days after the
originally scheduled date of the shareholders meetings.

     (g) Cornerstone shall request written consents for approval by the limited
partners of Cornerstone Partnership of each of the matters described in the
definition of Cornerstone Partner Approvals. Cornerstone hereby agrees to vote
in favor of or consent to, as applicable, the Partnership Merger, to the extent
approval thereof is required by the Cornerstone Partnership Agreement.
Cornerstone shall recommend to the limited partners of Cornerstone Partnership
that they approve such matters. EOP shall request written consents, if any is
required, by the limited partners, of EOP Partnership of each of the matters
described in the definition of EOP Partner Approvals. EOP hereby agrees to vote,
if any is required, in favor of such matters and to recommend to the limited
partners of EOP Partnership that they approve such matters.

     5.2 Access to Information; Confidentiality. Subject to the requirements of
confidentiality agreements with third parties in existence on the date hereof,
each of the parties shall, and shall cause each of its Subsidiaries (and, in the
case of Cornerstone, the Cornerstone Non-controlled Subsidiary) to, afford to
the other parties and to the officers, employees, accountants, counsel,
financial advisors and other representatives of such other parties, reasonable
access during normal business hours prior to the Effective Time to all their
respective properties, books, contracts, commitments, personnel and records and,
during such period, each of the parties shall, and shall cause each of its
Subsidiaries (and, in the case of Cornerstone, the Cornerstone Non-controlled
Subsidiary) to, furnish promptly to the other parties (a) a copy of each report,
schedule, registration statement and other document filed by it during such
period pursuant to the requirements of federal or state securities laws and (b)
all other information concerning its business, properties and personnel as such
other party may reasonably request. Each of the parties shall, and shall cause
its Subsidiaries (and in the case of Cornerstone, the Cornerstone Non-controlled
Subsidiary) to, use commercially reasonable efforts to cause its officers,
employees, accountants, counsel, financial advisors and other representatives
and affiliates to, hold any nonpublic information in confidence in accordance
with the Confidentiality Agreement, which shall remain in full force and effect
pursuant to the terms thereof, notwithstanding the execution and delivery of
this Agreement or the termination hereof.

     5.3 Commercially Reasonable Efforts; Notification.

     (a) Subject to the terms and conditions herein provided, each of the
parties shall: (i) use commercially reasonable efforts to cooperate with one
another in (A) determining which filings are required to be made prior to the
Effective Time with, and which consents, approvals, permits or authorizations
are required to be obtained prior to the Effective Time from, governmental or
regulatory authorities of the United States, the several states and foreign
jurisdictions and any third parties in connection with the execution and
delivery of this Agreement, and the consummation of the transactions
contemplated hereby, including, without limitation, any filing under the HSR
Act, and (B) timely making all such filings and timely seeking all such
consents, approvals, permits and authorizations; (ii) use commercially
reasonable efforts (other than the payment of money which is not contractually
required to be paid) to obtain in writing any consents required from third
parties to effectuate the Mergers, such consents to be in form reasonably
satisfactory to each of the parties (including, without limitation, taking the
actions contemplated under Schedule 5.3(a)(ii)); and (iii) use commercially
reasonable efforts to take, or cause to be taken, all other action and do, or
cause to be done, all other things necessary, proper or appropriate to
consummate and make effective the transactions contemplated by this Agreement.
If at

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any time after the Effective Time any further action is necessary or desirable
to carry out the purpose of this Agreement, each party shall take all such
necessary action.

     (b) Cornerstone and Cornerstone Partnership shall use commercially
reasonable efforts to obtain from PricewaterhouseCoopers LLP access to all work
papers relating to audits of Cornerstone and Cornerstone Partnership performed
by PricewaterhouseCoopers LLP, and the continued cooperation of
PricewaterhouseCoopers LLP with regard to the preparation of consolidated
financial statements for the Surviving Trust.

     (c) Cornerstone and Cornerstone Partnership shall give prompt notice to EOP
and EOP Partnership, and EOP and EOP Partnership shall give prompt notice to
Cornerstone and Cornerstone Partnership, (i) if any representation or warranty
made by it contained in this Agreement that is qualified as to materiality
becomes untrue or inaccurate in any respect or any such representation or
warranty that is not so qualified becomes untrue or inaccurate in any material
respect or (ii) of the failure by it to comply with or satisfy in any material
respect any covenant, condition or agreement to be complied with or satisfied by
it under this Agreement; provided, however, that no such notification shall
affect the representations, warranties, covenants or agreements of the parties
or the conditions to the obligations of the parties under this Agreement.

     5.4 Tax Matters.

     (a) Each of EOP and Cornerstone shall use its commercially reasonable
efforts before and after the Effective Time to cause the Merger to qualify as a
reorganization under the provisions of Sections 368(a) of the Code and to obtain
the opinions of counsel referred to in Sections 6.2(e) and 6.3(e).

     (b) Unless, and only to the extent, expressly provided otherwise in the Tax
Protection Agreements described in Schedule 2.18(j) to the Cornerstone
Disclosure Letter to be assumed by EOP pursuant to Section 5.14, EOP Partnership
and its Subsidiaries shall use the "traditional method" under Treasury
Regulations Section 1.704-3(b) for purposes of making allocations under Section
704(c) of the Code with respect to the properties of Cornerstone Partnership and
its Subsidiaries acquired in the Partnership Merger to take into account
differences as of the Effective Time of the Partnership Merger between the
adjusted tax bases of such properties and their respective fair market values
(referred to as the "Section 704(c) values"), with no curative allocations to
offset the effect of the "ceiling rule." EOP Partnership and Cornerstone
Partnership shall negotiate in good faith to agree upon the "Section 704(c)
values" of the properties of Cornerstone Partnership and its Subsidiaries as of
the Effective Time of the Partnership Merger. Unless, and only to the extent,
expressly provided otherwise in the Tax Protection Agreements described in
Schedule 2.18(j) to the Cornerstone Disclosure Letter to be assumed by EOP
pursuant to Section 5.14, EOP Partnership and its Subsidiaries shall determine
in their reasonable discretion the method to be used for allocating "excess
nonrecourse liabilities" of EOP Partnership pursuant to Treasury Regulations
Section 1.752-3(a)(3) following the Closing Date, provided that (i) EOP
Partnership shall not use with respect to the former limited partners in
Cornerstone Partnership a method that is less favorable than the method used by
EOP Partnership with respect to the other limited partners of EOP Partnership
who are not parties to an express agreement specifying a particular method to be
used for such purposes, and (ii) in the case of a Cornerstone Partner who, prior
to the Partnership Merger, had been specially allocated a portion of a
Cornerstone Partnership nonrecourse liability secured by a property with respect
to which such Cornerstone Partner has a built-in gain under Section 704(c) of
the Code to take into account such Cornerstone Partner's share of such built-in
gain that was not taken into account in making the allocation of such liability
under Treasury Regulation Section 1.752-3(a)(2), EOP Partnership shall continue
such method of allocating such liability following the Merger.

     (c) EOP Partnership shall amend the EOP Partnership Agreement at or prior
to the Effective Time of the Partnership Merger to incorporate provisions
relating to the restoration of deficit capital accounts that will permit each
limited partner of Cornerstone Partnership who has entered into an agreement
with Cornerstone Partnership prior to the Partnership Merger relating to the
restoration of deficit capital accounts to continue to be obligated to restore
any deficit in its capital account in EOP Partnership in an amount that is not
less than the maximum potential amount that such limited partner was obligated
to
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restore to Cornerstone Partnership immediately prior to the Effective Time of
the Partnership Merger. Such deficit capital account restoration provisions to
be incorporated in the EOP Partnership Agreement shall be in substantially the
form of the deficit capital account restoration provisions incorporated in the
Cornerstone Partnership Agreement pursuant to the Fourth Amendment of Agreement
of Limited Partnership of Cornerstone Properties Limited Partnership, with such
changes thereto as shall be approved by Cornerstone Partnership, provided,
however, that except as set forth in the next sentence below or in the Tax
Protection Agreements listed on Schedule 2.18(j) to the Cornerstone Disclosure
Letter, no Cornerstone Partner shall have the right to increase the amount of
its deficit restoration obligation following the Partnership Merger to an amount
in excess of the maximum potential amount that such limited partner was
obligated to restore to Cornerstone Partnership immediately prior to the
Effective Time of the Partnership Merger. Notwithstanding the proviso in the
immediately preceding sentence, the Cornerstone Partners as a group shall have
the right, at any time following the Merger and without the consent of EOP, to
increase their maximum potential deficit restoration obligations as a group (as
determined immediately prior to the Merger) by an aggregate amount of up to $50
million, reduced by the amount of any increase therein that shall occur under
the Cornerstone Partnership Agreement during the period commencing on the date
hereof and ending immediately prior to the Effective Time of the Partnership
Merger. Any election to increase or to take on a deficit restoration obligation
election permitted under a Tax Protection Agreement listed on Schedule 2.18(j)
to the Cornerstone Disclosure Letter shall not be taken into account in applying
the foregoing $50 million limitation. Notwithstanding any the foregoing, EOP
Partnership shall not be obligated to maintain any level of partnership recourse
debt in excess of the amounts otherwise specifically required to be maintained
under the Tax Protection Agreements listed on Schedule 2.18(j) to the
Cornerstone Disclosure Letter. In addition, the amendment to the EOP Partnership
Agreement will contain a provision to the effect that (i) EOP Partnership will
consider in good faith a request from a former Cornerstone Partner to increase
the amount of its deficit restoration obligation from time to time after the
Partnership Merger if the former Cornerstone Partner shall provide information
from its professional tax advisor satisfactory to EOP Partnership showing that,
in the absence of such an increase, such former Cornerstone Partner would not
likely be allocated from EOP Partnership sufficient indebtedness under Section
752 of the Code and the at-risk provisions under Section 465 of the Code to
avoid the recognition of gain (other than gain required to be recognized by
reason of actual cash distributions from EOP Partnership following the
Partnership Merger); (ii) EOP Partnership and its professional tax advisors will
cooperate in good faith with the former Cornerstone Partner and its professional
tax advisor to provide such information regarding the allocation of the EOP
Partnership liabilities and the nature of such liabilities as is reasonably
necessary in order to determine the former Cornerstone Partner's adjusted tax
basis in its EOP OP Units and at-risk amount; (iii) in deciding whether or not
to grant such request, EOP Partnership shall be entitled to take into account
all factors related to EOP Partnership, including, without limitation, the
existing and anticipated debt structure of EOP Partnership, the tax situations
of other holders of EOP OP Units and the effect that such a deficit restoration
commitment might have on their tax situation, and the anticipated term long term
business needs of EOP; (iv) in no event shall EOP Partnership be required to
incur additional indebtedness that would be considered a "recourse liability"
for purposes of Section 752 of the Code to facilitate such additional deficit
capital account restoration commitments; and (v) EOP Partnership's only
obligation shall be to act in good faith, and a former Cornerstone Partner's
exclusive remedy under such provision would be an action for specific
performance, with no entitlement to monetary damages.

     5.5 Public Announcements. Each party will consult with each other party
before issuing, and provide each other the opportunity to review and comment
upon, any press release or other written public statements, including, without
limitation, any press release or other written public statement which address in
any manner the transactions contemplated by this Agreement, and shall not issue
any such press release or make any such written public statement prior to such
consultation, except as may be required by applicable law, court process or by
obligations pursuant to any listing agreement with any national securities
exchange. The parties agree that the initial press release to be issued with
respect to the transactions contemplated by this Agreement will be in the form
agreed to by the parties prior to the execution of this Agreement.

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     5.6 Listing. EOP shall use commercially reasonable efforts to cause the EOP
Common Shares to be issued in the Merger, and the EOP Common Shares reserved for
issuance upon redemption of EOP OP Units issued in the Partnership Merger, to be
approved for listing on the NYSE, subject to official notice of issuance, prior
to the Effective Time.

     5.7 Transfer and Gains Taxes. Each party shall cooperate in the
preparation, execution and filing of all returns, questionnaires, applications
or other documents regarding any real property transfer or gains, sales, use,
transfer, value added stock transfer and stamp taxes, any transfer, recording,
registration and other fees and any similar taxes which become payable in
connection with the transactions contemplated by this Agreement (together with
any related interests, penalties or additions to tax, "Transfer and Gains
Taxes"). From and after the Effective Time, EOP shall pay or cause EOP Operating
Partnership, as appropriate, to pay or cause to be paid, without deduction or
withholding from any amounts payable to the holders of EOP Common Shares or EOP
OP Units, as applicable, all Transfer and Gains Taxes (which term shall not in
any event be construed to include for these purposes any Tax imposed under the
Code).

     5.8 Benefit Plans and Other Employee Arrangements.

     (a) Benefit Plans. After the Effective Time, all employees of Cornerstone
who are employed by the Surviving Trust shall, at the option of the Surviving
Trust, either continue to be eligible to participate in an "employee benefit
plan", as defined in Section 3(3) of ERISA, of Cornerstone which is, at the
option of the Surviving Trust, continued by the Surviving Trust, or
alternatively shall be eligible to participate in the same manner as other
similarly situated employees of the Surviving Trust who were formerly employees
of EOP in any "employee benefit plan," as defined in Section 3(3) of ERISA,
sponsored or maintained by the Surviving Trust after the Effective Time. With
respect to each such employee benefit plan, service with Cornerstone or any
Cornerstone Subsidiary (as applicable) and the predecessor of any of them shall
be included for purposes of determining eligibility to participate, vesting (if
applicable) and determination of the level of entitlement to, benefits under
such employee benefit plan. EOP shall, or shall cause the Surviving Trust and
its Subsidiaries to, (i) waive all limitations, as to preexisting conditions
exclusions and waiting periods with respect to participation and coverage
requirements applicable to all employees of Cornerstone who are employed by the
Surviving Trust under any welfare plan that such employees may be eligible to
participate in after the Effective Time, other than limitations or waiting
periods that are already in effect with respect to such employees and that have
not been satisfied as of the Effective Time under any welfare plan maintained
for such employees immediately prior to the Effective Time, and (ii) provide
each such employee of Cornerstone who is employed by the Surviving Trust with
credit for any co-payments and deductibles paid prior to the Effective Time in
satisfying any applicable deductible or out-of-pocket requirements under any
welfare plans that such employees are eligible to participate in after the
Effective Time.

     (b) Stock Option and Restricted Stock Plans. The stock option plans or
programs of Cornerstone and the restricted stock plans or programs of
Cornerstone shall be discontinued.

     (c) Cornerstone Stock Options. As of the Effective Time, each outstanding
Cornerstone Stock Option shall, whether or not then vested or exercisable,
effective as of the Effective Time, become fully exercisable and vested and each
such Cornerstone Stock Option shall be automatically converted at the Effective
Time into an option (a "Substituted Option") to purchase a number of shares of
EOP Common Shares equal to the number of shares of Cornerstone Common Stock that
could have been purchased (assuming full vesting) under such Cornerstone Stock
Option multiplied by 0.7009 (rounded down to the nearest whole number of shares
of Cornerstone Common Stock) at an exercise price per share of EOP Common Shares
equal to the per-share option exercise price specified in the Cornerstone Stock
Option divided by 0.7009 (rounded up to the nearest whole cent). Such
Substituted Option shall otherwise be subject to the same terms and conditions
as such Cornerstone Stock Option. For purposes of expiration and otherwise, the
date of grant of the Substituted Option shall be the date on which the
corresponding Cornerstone Stock Option was granted. If the holder of such
Cornerstone Stock Option surrenders such option after the Effective Time and
prior to 11:59 p.m., Eastern Time, on the second business day immediately
following the Effective Time, EOP shall, subject to reduction for required
withholding taxes,

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pay to each such holder of Cornerstone Stock Options an amount in cash in
respect thereof equal to the product of (i) the excess, if any, of $18.00 over
the exercise price of such Cornerstone Stock Option and (ii) the number of
shares of Cornerstone Common Stock subject thereto. At the Effective Time, (i)
all references in the related stock option agreements to Cornerstone shall be
deemed to refer to EOP and (ii) EOP shall assume all of Cornerstone's
obligations with respect to Cornerstone Stock Options as so amended. As promptly
as reasonably practicable after the Effective Time, EOP shall issue to each
holder of an outstanding Cornerstone Stock Option a document evidencing the
foregoing assumption by EOP.

     In respect of each Cornerstone Stock Option assumed by EOP and converted
into a Substituted Option, and the EOP Common Shares underlying such Substituted
Option, EOP shall, as soon as practicable after the Effective Time, file and
keep current a Registration Statement on Form S-8 or other appropriate
registration statement for as long as Substituted Options remain outstanding.

     (d) Restricted Stock. All unvested shares of restricted stock of
Cornerstone set forth in Schedule 5.8(d) of the Cornerstone Disclosure Schedule,
shall, by virtue of this Agreement and without further action of Cornerstone,
EOP or the holder of such shares of restricted stock, to the extent required in
the plan, agreement or instrument pursuant to which such shares of restricted
stock were granted, vest and become free of all restrictions immediately prior
to the Effective Time and shall be converted into the Merger Consideration
pursuant to Section 1.10.

     (e) Retention Agreements and Retention Plan. From and after the Effective
Time, EOP or EOP Partnership shall assume the obligations of Cornerstone under
the Retention Agreements and the Cornerstone retention plan set forth on
Schedule 5.9(e) of the Cornerstone Disclosure Letter. The transactions
contemplated by this Agreement shall be deemed a "Change in Control" for
purposes of such agreements, and EOP shall perform the obligations of
Cornerstone under such agreements and plan in accordance with the terms thereof.

     (f) Withholding. To the extent required by applicable law, Cornerstone
shall require each employee who exercises a Cornerstone Stock Option or who
receives Cornerstone Common Stock pursuant to any existing commitment to pay to
Cornerstone in cash or Cornerstone Common Stock an amount sufficient to satisfy
in full Cornerstone's obligation to withhold Taxes incurred by reason of such
exercise or issuance (unless and to the extent such withholding is satisfied
pursuant to the provision regarding withholding in Section 1.15(c)).

     5.9 Indemnification.

     (a) From and after the Effective Time, EOP and EOP Partnership
(collectively, the "Indemnifying Parties") shall provide exculpation and
indemnification for each person who is now or has been at any time prior to the
date hereof or who becomes prior to the Effective Time of the Merger, an officer
or director of Cornerstone or any Cornerstone Subsidiary (the "Indemnified
Parties") which is the same as the exculpation and indemnification provided to
the Indemnified Parties by Cornerstone and the Cornerstone Subsidiaries
immediately prior to the Effective Time of the Merger in its charter, Bylaws or
in its partnership, operating or similar agreement, as in effect on the date
hereof.

     (b) In addition to the rights provided in Section 5.9(a) above, in the
event of any threatened or actual claim, action, suit, proceeding or
investigation, whether civil, criminal or administrative, including, without
limitation, any action by or on behalf of any or all security holders of
Cornerstone or EOP, or any Cornerstone Subsidiary or EOP Subsidiary, or by or in
the right of Cornerstone or EOP, or any Cornerstone Subsidiary or EOP
Subsidiary, or any claim, action, suit, proceeding or investigation in which any
person who is now, or has been, at any time prior to the date hereof, or who
becomes prior to the Effective Time of the Merger, an officer, employee or
director of Cornerstone or any Cornerstone Subsidiary (the "Indemnification
Parties") is, or is threatened to be, made a party based in whole or in part on,
or arising in whole or in part out of, or pertaining to (i) the fact that he is
or was an officer, employee or director of Cornerstone or any of the Cornerstone
Subsidiaries or any action or omission by such person in his capacity as a
director, or (ii) this Agreement or the transactions contemplated by this
Agreement, whether in any case asserted or arising before or after the Effective
Time of the Merger, the

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Indemnifying Parties shall, from and after the Effective Time of the Merger,
indemnify and hold harmless, as and to the full extent permitted by applicable
law, each Indemnification Party against any losses, claims, liabilities,
expenses (including reasonable attorneys' fees and expenses), judgments, fines
and amounts paid in settlement in accordance herewith in connection with any
such threatened or actual claim, action, suit, proceeding or investigation. Any
Indemnification Party proposing to assert the right to be indemnified under this
Section 5.9(b) shall, promptly after receipt of notice of commencement of any
action against such Indemnification Party in respect of which a claim is to be
made under this Section 5.9(b) against the Indemnifying Parties, notify the
Indemnifying Parties of the commencement of such action, enclosing a copy of all
papers served; provided, however, that the failure to provide such notice shall
not affect the obligations of the Indemnifying Parties except to the extent such
failure to notify materially prejudices the Indemnifying Parties' ability to
defend such claim, action, suit, proceeding or investigation; and provided,
further, however, that, in the case of any action pending at the Effective Time
of the Merger, notification pursuant to this Section 5.9(b) shall be received by
EOP prior to such Effective Time. If any such action is brought against any of
the Indemnification Parties and such Indemnification Parties notify the
Indemnifying Parties of its commencement, the Indemnifying Parties will be
entitled to participate in and, to the extent that they elect by delivering
written notice to such Indemnification Parties promptly after receiving notice
of the commencement of the action from the Indemnification Parties, to assume
the defense of the action and after notice from the Indemnifying Parties to the
Indemnification Parties of their election to assume the defense, the
Indemnifying Parties will not be liable to the Indemnification Parties for any
legal or other expenses except as provided below. If the Indemnifying Parties
assume the defense, the Indemnifying Parties shall have the right to settle such
action without the consent of the Indemnification Parties; provided, however,
that the Indemnifying Parties shall be required to obtain such consent (which
consent shall not be unreasonably withheld) if the settlement includes any
admission of wrongdoing on the part of the Indemnification Parties or any decree
or restriction on the Indemnification Parties; provided, further, that no
Indemnifying Parties, in the defense of any such action shall, except with the
consent of the Indemnification Parties (which consent shall not be unreasonably
withheld), consent to entry of any judgment or enter into any settlement that
does not include as an unconditional term thereof the giving by the claimant or
plaintiff to such Indemnification Parties of a release from all liability with
respect to such action. The Indemnification Parties will have the right to
employ their own counsel in any such action, but the fees, expenses and other
charges of such counsel will be at the expense of such Indemnification Parties
unless (i) the employment of counsel by the Indemnification Parties has been
authorized in writing by the Indemnifying Parties, (ii) the Indemnification
Parties have reasonably concluded (based on written advice of counsel to the
Indemnification Parties) that there may be legal defenses available to them that
are different from or in addition to and inconsistent with those available to
the Indemnifying Parties, (iii) a conflict or potential conflict exists (based
on written advice of counsel to the Indemnification Parties) between the
Indemnification Parties and the Indemnifying Parties (in which case the
Indemnifying Parties will not have the right to direct the defense of such
action on behalf of the Indemnification Parties) or (iv) the Indemnifying
Parties have not in fact employed counsel to assume the defense of such action
within a reasonable time after receiving notice of the commencement of the
action from the Indemnification Parties, in each of which cases the reasonable
fees, disbursements and other charges of counsel will be at the expense of the
Indemnifying Parties and shall promptly be paid by each Indemnifying Party as
they become due and payable in advance of the final disposition of the claim,
action, suit, proceeding or investigation to the fullest extent and in the
manner permitted by law; provided, however, that in no event shall any
contingent fee arrangement be considered reasonable. Notwithstanding the
foregoing, the Indemnifying Parties shall not be obligated to advance any
expenses or costs prior to receipt of an undertaking by or on behalf of the
Indemnification Party to repay any expenses advanced if it shall ultimately be
determined that the Indemnification Party is not entitled to be indemnified
against such expense. It is understood that the Indemnifying Parties shall not,
in connection with any proceeding or related proceedings in the same
jurisdiction, be liable for the reasonable fees, disbursements and other charges
of more than one separate firm admitted to practice in such jurisdiction at any
one time for all such Indemnification Parties unless (a) the employment of more
than one counsel has been authorized in writing by the Indemnifying Parties, (b)
any of the Indemnification Parties have reasonably concluded
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(based on written advice of counsel to the Indemnification Parties) that there
may be legal defenses available to them that are different from or in addition
to and inconsistent with those available to other Indemnification Parties or (c)
a conflict or potential conflict exists (based on written advice of counsel to
the Indemnification Parties) between any of the Indemnification Parties and the
other Indemnification Parties, in each case of which the Indemnifying Parties
shall be obligated to pay the reasonable fees and expenses of such additional
counsel or counsels. Notwithstanding anything to the contrary set forth in this
Agreement, the Indemnifying Parties (i) shall not be liable for any settlement
effected without their prior written consent and (ii) shall not have any
obligation hereunder to any Indemnification Party to the extent that a court of
competent jurisdiction shall determine in a final and non-appealable order that
such indemnification is prohibited by applicable law. In the event of a final
and non-appealable determination by a court that any payment of expenses is
prohibited by applicable law, the Indemnification Parties shall promptly refund
to the Indemnifying Parties the amount of all such expenses theretofore advanced
pursuant hereto.

     (c) At or prior to the Effective Time of the Merger, EOP shall purchase
directors' and officers' liability insurance covering acts or omissions
occurring prior to the Effective Time of the Merger for a period of six years
with respect to those persons who are currently covered by Cornerstone's
directors' and officers' liability insurance policy on terms with respect to
such coverage and amount no less favorable to Cornerstone's directors and
officers currently covered by such insurance than those of such policy in effect
on the date hereof.

     (d) This Section 5.9 is intended for the irrevocable benefit of, and to
grant third-party rights to, the Indemnified Parties, the Indemnification
Parties and their successors, assigns and heirs and shall be binding on all
successors and assigns of EOP and EOP Operating Partnership. Each of the
Indemnified Parties and the Indemnification Parties shall be entitled to enforce
the covenants contained in this Section 5.9 and EOP and EOP Partnership
acknowledge and agree that each Indemnified Party and Indemnification Party
would suffer irreparable harm and that no adequate remedy at law exists for a
breach of such covenants and such Indemnified Party or such Indemnification
Party shall be entitled to injunctive relief and specific performance in the
event of any breach of any provision in this Section 5.9.

     (e) If EOP or EOP Partnership or any of its respective successors or
assigns (i) consolidates with or merges into any other person and shall not be
the continuing or surviving corporation or entity of such consolidation or
merger or (ii) transfers all or substantially all of its properties and assets
to any person, then, and in each such case the successors and assigns of such
entity shall assume the obligations set forth in this Section 5.9, which
obligations are expressly intended to be for the irrevocable benefit of, and
shall be enforceable by, each director and officer covered hereby.

     5.10 Declaration of Dividends and Distributions. From and after the date of
this Agreement, neither Cornerstone nor EOP shall make any dividend or
distribution to its respective stockholders or shareholders without the prior
written consent of the other party; provided, however, the written consent of
the other party shall not be required for the authorization and payment of (a)
distributions at their respective stated dividend or distribution rates with
respect to EOP Preferred Shares or Cornerstone 7% Preferred Stock, (b) quarterly
distribution with respect to the Cornerstone Common Stock of up to $0.24 per
share for the quarter ending March 31, 2000 and up to $0.31 per share thereafter
and (c) quarterly distributions with respect to the EOP Common Share of up to
$0.42 per share for the quarter ending March 31, 2000 and for each quarter
thereafter; provided, however, except for the record date previously set on
January 31, 2000, the record date for each distribution with respect to the
Cornerstone Common Stock shall be the same date as the record date for the
quarterly distribution for the EOP Common Shares, as provided to Cornerstone by
notice not less than twenty (20) business days prior to the record date for any
quarterly EOP distribution. From and after the date of this Agreement,
Cornerstone Partnership shall not make any distribution to the holders of
Cornerstone OP Units except a distribution per Cornerstone OP Unit in the same
amount as a dividend per share of Cornerstone Common Stock permitted pursuant to
this Section 5.10, with the same record and payment dates as such dividend on
the Cornerstone Common Stock. The foregoing restrictions shall not apply,
however, to the extent a distribution (or an increase in a distribution) by
Cornerstone or EOP is necessary for Cornerstone or EOP, as applicable, to
maintain
                                      A-48
<PAGE>   168

REIT status, avoid the incurrence of any taxes under Section 857 of the Code,
avoid the imposition of any excise taxes under Section 4981 of the Code, or
avoid the need to make one or more extraordinary or disproportionately larger
distributions to meet any of the three preceding objectives.

     5.11 Transfer of Non-Controlled Subsidiary Voting Shares. At the Closing
and pursuant to the Stock Purchase Agreement, each of the holders of voting
capital stock of the Cornerstone Non-controlled Subsidiary (other than
Cornerstone Partnership, to the extent it owns any such voting capital stock)
shall transfer to EOP NCS Sub or such person or persons as EOP NCS Sub shall
designate by written notice delivered to them prior to the Closing, all of the
shares of each such Company owned by them, constituting all the outstanding
shares of such companies which are not owned by Cornerstone Partnership, for an
aggregate consideration in an amount equal to the purchase price set forth in
the Stock Purchase Agreement. EOP shall use commercially reasonable efforts to
cause EOP NCS Sub to perform its obligations under the Stock Purchase Agreement

     5.12 Notices. EOP shall provide such notice to its preferred shareholders
of the Merger as is required under Maryland law or the EOP Declaration of Trust.

     5.13 Resignations. On the Closing Date, Cornerstone shall cause the
directors and officers of Cornerstone and of each of the Cornerstone
Subsidiaries to submit their resignations from such positions, effective as of
the Effective Time of the Merger.

     5.14 Assumption of Existing Tax Protection Agreements. Effective as of the
Effective Time of the Partnership Merger, EOP and EOP Partnership shall assume
the obligations of Cornerstone, Cornerstone Partnership and/or the applicable
Cornerstone Subsidiary, as the case may be, under the Tax Protection Agreements
as described in Schedule 2.18(j) to the Cornerstone Disclosure Letter.
Immediately prior to the Effective Time of the Partnership Merger, EOP and EOP
Partnership shall enter into agreements with Cornerstone and Cornerstone
Partnership, for the benefit of and enforceable by the individuals and entities
who are intended to be protected by the provisions of the Tax Protection
Agreements, confirming such assumption effective as of the Effective Time of the
Partnership Merger.

     5.15 EOP Partnership Agreement. At the Closing, EOP Partnership shall
assume and perform any obligations that Cornerstone Partnership or any
Cornerstone Subsidiary has immediately prior to the Effective Time to issue
securities in accordance with the terms of any partnership or other agreement to
which Cornerstone Partnership or such Cornerstone Subsidiary is a party and that
are described on Schedule 5.15, in the same manner and to the same extent that
Cornerstone Partnership or such Cornerstone Subsidiary would be required to
perform such obligation if no Merger had been consummated.

     5.16 Registration Rights Agreements. At the Closing, (a) Cornerstone shall
assign and EOP shall assume by appropriate instrument the Registration Rights
Agreements described on Schedule 5.16 to the Cornerstone Disclosure Letter, and
(b) Cornerstone shall assign and EOP shall assume the obligations of Cornerstone
under the Amended and Restated Registration Rights and Voting Agreement dated
December 16, 1998 between Cornerstone, PGGM and Dutch Institutional Holding
Company, Inc., as and to the extent set forth in PGGM's Cornerstone Voting
Agreement.

     5.17 Cornerstone Convertible Promissory Note. Following the Effective Time
of the Mergers, the holders of the Cornerstone Convertible Promissory Note shall
have the right to obtain upon the exercise of its conversion rights pursuant to
such Cornerstone Convertible Promissory Note, in lieu of each share of
Cornerstone Common Stock theretofor issuable upon exercise of such conversion
rights, EOP Common Shares that it would have owned immediately after the Merger
if it had converted the Cornerstone Convertible Promissory Note immediately
before the Effective Time of the Merger.

                                      A-49
<PAGE>   169

                                   ARTICLE 6

                                   CONDITIONS

     6.1 Conditions to Each Party's Obligation to Effect the Mergers. The
obligations of each party to effect the Mergers and to consummate the other
transactions contemplated by this Agreement to occur on the Closing Date shall
be subject to the fulfillment at or prior to the Closing Date of the following
conditions:

     (a) Shareholder and Partner Approvals. The Cornerstone Stockholder
Approvals, the EOP Shareholder Approvals and the Partner Approvals shall have
been obtained.

     (b) HSR Act. The waiting period (and any extension thereof) applicable to
the Partnership Merger, the Merger or the transactions contemplated by the Stock
Purchase Agreement under the HSR Act, if applicable to the Partnership Merger,
the Merger or and the transactions contemplated by the Stock Purchase Agreement,
shall have expired or been terminated.

     (c) Listing of Shares. The NYSE shall have approved for listing the EOP
Common Shares to be issued in the Merger and the EOP Common Shares reserved for
issuance upon redemption of EOP OP Units issued in the Partnership Merger,
subject to official notice of issuance.

     (d) Form S-4. The Form S-4 shall have become effective under the Securities
Act and shall not be the subject of any stop order or proceedings by the SEC
seeking a stop order.

     (e) No Injunctions or Restraints. No temporary restraining order,
preliminary or permanent injunction or other order issued by any court of
competent jurisdiction or other legal restraint or prohibition preventing the
consummation of the Mergers or any of the other transactions contemplated hereby
shall be in effect.

     (f) Blue Sky Laws. EOP and EOP Partnership shall have received all state
securities or "blue sky" permits and other authorizations necessary to issue the
EOP Common Shares and EOP OP Units issuable in the Mergers.

     6.2 Conditions to Obligations of EOP and EOP Partnership. The obligations
of EOP and EOP Partnership to effect the Mergers and to consummate the other
transactions contemplated to occur on the Closing Date are further subject to
the following conditions, any one or more of which may be waived by EOP:

     (a) Representations and Warranties. Each of the representations and
warranties of Cornerstone and Cornerstone Partnership set forth in this
Agreement, disregarding all qualifications and exceptions contained therein
relating to materiality or Cornerstone Material Adverse Effect, shall be true
and correct as of the date of this Agreement and as of the Closing Date as
though made on and as of the Closing Date (except to the extent that such
representations and warranties are expressly limited by their terms to another
date, in which case such representations and warranties shall be true and
correct as of such other date), except where the failure of such representations
and warranties to be true and correct would not, individually or in the
aggregate, reasonably be expected to have a Cornerstone Material Adverse Effect;
and EOP shall have received a certificate (which certificate may be qualified by
Knowledge to the same extent as the representations and warranties of
Cornerstone and Cornerstone Partnership contained herein are so qualified)
signed on behalf of Cornerstone by the chief executive officer or the chief
financial officer of Cornerstone, in such capacity, to such effect.

     (b) Performance of Obligations of Cornerstone and Cornerstone
Partnership.  Cornerstone and Cornerstone Partnership shall have performed in
all material respects all obligations required to be performed by them under
this Agreement at or prior to the Effective Time, and EOP shall have received a
certificate signed on behalf of Cornerstone by the chief executive officer or
the chief operating officer of Cornerstone, in such capacity, to such effect.

                                      A-50
<PAGE>   170

     (c) Material Adverse Change. Since the date of this Agreement, there shall
have been no Cornerstone Material Adverse Change and EOP shall have received a
certificate of the chief executive officer or chief operating officer of
Cornerstone, in such capacity, certifying to such effect.

     (d) Tax Opinions Relating to REIT Status and Partnership Status. EOP shall
have received (i) an opinion of King & Spalding or other counsel to Cornerstone
reasonably satisfactory to EOP, dated as of the Closing Date, to the effect
that, commencing with its taxable year ended December 31, 1997, (x) Cornerstone
was organized and has operated in conformity with the requirements for
qualification as a REIT under the Code, and (y) Cornerstone Partnership has been
during and since December 23, 1997, and continues to be, treated for federal
income tax purposes as a partnership and not as a corporation or association
taxable as a corporation (with customary exceptions, assumptions and
qualifications and based upon customary representations) and (ii) an opinion of
Hogan & Hartson L.L.P. or other counsel to EOP reasonably satisfactory to
Cornerstone, dated as of the Closing Date, to the effect that, commencing with
its taxable year ended December 31, 1997, EOP was organized and has operated in
conformity with the requirements for qualification as a REIT under the Code and
that, after giving effect to the Merger, EOP's proposed method of operation will
enable it to continue to meet the requirements for qualification and taxation as
a REIT under the Code (with customary exceptions, assumptions and qualifications
and based upon customary representations and based upon and subject to the
opinion of counsel to Cornerstone described in clause (i) above).

     (e) Tax Opinion Relating to the Merger. EOP shall have received an opinion
dated the Closing Date from Hogan & Hartson L.L.P. or other counsel reasonably
satisfactory to EOP, based upon customary certificates and letters, which
letters and certificates are to be in a form to be agreed upon by the parties
and dated the Closing Date, to the effect that the Merger will qualify as a
reorganization under the provisions of Section 368(a) of the Code.

     (f) "Comfort" Letter. EOP and EOP Partnership shall have received a
"comfort" letter from PricewaterhouseCoopers LLP, as described in Section
5.1(b).

     (g) Noncompetition Agreements. Each of William Wilson III and John S. Moody
shall have entered into noncompetition and confidentiality agreements
substantially in the forms set forth in Schedule 6.2(g) hereto.

     6.3 Conditions to Obligations of Cornerstone and Cornerstone
Partnership.  The obligations of Cornerstone and Cornerstone Partnership to
effect the Mergers and to consummate the other transactions contemplated to
occur on the Closing Date is further subject to the following conditions, any
one or more of which may be waived by Cornerstone:

     (a) Representations and Warranties. Each of the representations and
warranties of EOP and EOP Partnership set forth in this Agreement, disregarding
all qualifications and exceptions contained therein relating to materiality or
EOP Material Adverse Effect, shall be true and correct as of the date of this
Agreement and as of the Closing Date as though made on and as of the Closing
Date (except to the extent that such representations and warranties are
expressly limited by their terms to another date, in which case such
representations and warranties shall be true and correct as of such other date),
except where the failure of such representations and warranties to be true and
correct would not, individually or in the aggregate, reasonably be expected to
have a EOP Material Adverse Effect; and Cornerstone shall have received a
certificate (which certificate may be qualified by Knowledge to the same extent
as the representations and warranties of EOP and EOP Partnership contained
herein are so qualified) signed on behalf of EOP by the chief executive officer
or the chief financial officer of EOP, in such capacity, to such effect.

     (b) Performance of Obligations of EOP and EOP Partnership. EOP and EOP
Partnership shall have performed in all material respects all obligations
required to be performed by it under this Agreement at or prior to the Effective
Time, and Cornerstone shall have received a certificate of EOP signed on behalf
of EOP by a duly authorized executive officer of EOP, in such capacity, to such
effect.

                                      A-51
<PAGE>   171

     (c) Material Adverse Change. Since the date of this Agreement, there shall
have been no EOP Material Adverse Change and Cornerstone shall have received a
certificate of a duly authorized executive officer of EOP, in such capacity,
certifying to such effect.

     (d) Tax Opinions Relating to REIT Status and Partnership Status.
Cornerstone shall have received the opinion of Hogan & Hartson L.L.P. or other
counsel to EOP reasonably satisfactory to Cornerstone, dated as of the Closing
Date, that, commencing with its taxable year ended December 31, 1997, (i) EOP
was organized and has operated in conformity with the requirements for
qualification as a REIT under the Code and that, after giving effect to the
Merger, EOP's proposed method of operation will enable it to continue to meet
the requirements for qualification and taxation as a REIT under the Code (with
customary exceptions, assumptions and qualifications and based upon customary
representations and based upon and subject to the opinion of counsel to
Cornerstone described in Section 6.2(d) of this Agreement), (ii) BeaMet was
organized and has operated in conformity with the requirements for qualification
as a REIT under the Code (with customary exceptions, assumptions and
qualifications and based upon customary representations), and (iii) EOP
Partnership has been during and since 1997, and continues to be, treated for
federal income tax purposes as a partnership and not as a corporation or
association taxable as a corporation (with customary exceptions, assumptions and
qualifications and based upon customary representations).

     (e) Tax Opinion Relating to the Merger. Cornerstone shall have received an
opinion dated the Closing Date from King & Spalding or other counsel reasonably
satisfactory to Cornerstone, based upon customary certificates and letters,
which letters and certificates are to be in a form to be agreed upon by the
parties and dated the Closing Date, to the effect that the Merger will qualify
as a reorganization under the provisions of Section 368(a) of the Code.

     (f) Proposed EOP Charter Amendment Relating to Domestically Controlled REIT
Status. The EOP Shareholders shall have approved the Proposed EOP Charter
Amendment Relating to Domestically Controlled REIT Status at the EOP
Shareholders Meeting.

     (g) "Comfort" Letter. Cornerstone and Cornerstone Partnership shall have
received a "comfort" letter from Ernst & Young LLP, as described in Section
5.1(b).

     (h) Certificate Regarding "Domestically Controlled REIT" Status to PGGM.
EOP shall have delivered to Stichting Pensioenfonds voor de Gezondheid,
Geestelijke en Maatschappelijke Belangen ("PGGM") a certification dated within
fifteen (15) days of the Closing Date that to EOP's knowledge, after reasonable
inquiry, EOP is a "domestically controlled REIT" within the meaning of Section
897(h)(4)(B) of the Code as of the date thereof. For purposes of such
certification, reasonable inquiry shall include (but not necessarily be limited
to) review of all Schedule 13D and 13G filings made under the Exchange Act with
the SEC with respect to EOP, all IRS Form 1042 filings made by or on behalf of
EOP, the list of EOP's registered shareholders as of a date within 60 days of
such certificate (and to the extent reasonably available, as of a date within 60
days of the end of each of 1997, 1998, and 1999), a list of "non-objecting
beneficial owners" of shares of EOP obtained as of a date within 60 days of such
certificate (and to the extent reasonably available, as of a date within 60 days
of the end of each of 1997, 1998, and 1999), and a report of a shareholder
tracking service obtained within 60 days of such certificate (together with such
other reports as are in the possession of EOP). Such certificate shall be
accompanied by copies of information that has been obtained or relied upon by
EOP for purposes of such certificate, provided that PGGM shall have executed an
agreement with EOP to treat such information as confidential and to use such
information solely for the purposes of evaluating the accuracy of such
certification.

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<PAGE>   172

                                   ARTICLE 7

                       TERMINATION, AMENDMENT AND WAIVER

     7.1 Termination. This Agreement may be terminated at any time prior to the
Effective Time of the Partnership Merger, whether such action occurs before or
after any of the Cornerstone Stockholder Approvals, the EOP Shareholder
Approvals or either of the Cornerstone Partner Approvals are obtained:

     (a) by mutual written consent duly authorized by the Board of Trustees of
EOP and the Board of Directors of Cornerstone;

     (b) by EOP, upon a breach of or failure to perform any representation,
warranty, covenant, obligation or agreement on the part of Cornerstone or
Cornerstone Partnership set forth in this Agreement, or if any representation or
warranty of Cornerstone or Cornerstone Partnership shall become untrue, in
either case such that the conditions set forth in Section 6.2(a) or Section
6.2(b), as the case may be, would be incapable of being satisfied by December
31, 2000 (or as otherwise extended);

     (c) by Cornerstone, upon a breach of any representation, warranty, covenant
obligation or agreement on the part of EOP or EOP Partnership set forth in this
Agreement, or if any representation or warranty of EOP or EOP Partnership shall
become untrue, in either case such that the conditions set forth in Section
6.3(a) or Section 6.3(b), as the case may be, would be incapable of being
satisfied by December 31, 2000 (or as otherwise extended);

     (d) by either EOP or Cornerstone, if any judgment, injunction, order,
decree or action by any Governmental Entity of competent authority preventing
the consummation of either of the Mergers shall have become final and
non-appealable;

     (e) by either EOP or Cornerstone, if the Mergers shall not have been
consummated before December 31, 2000; provided, however, that a party may not
terminate pursuant to this clause (e) if the terminating party shall have
breached in any material respect its obligations under this Agreement in any
manner that shall have caused either of the Mergers not to have been consummated
by such date;

     (f) by either EOP or Cornerstone (unless Cornerstone or Cornerstone
Partnership is in breach in any material respect of its obligations under
Section 5.1) if, upon a vote at a duly held Cornerstone Stockholders Meeting or
any adjournment thereof, the Cornerstone Stockholder Approvals shall not have
been obtained as contemplated by Section 5.1 or if the Cornerstone Partner
Approvals have not been obtained as contemplated by Section 5.1;

     (g) by either Cornerstone or EOP (unless EOP or EOP Partnership is in
breach in any material respect of its obligations under Section 5.1 if, upon a
vote at a duly held EOP Shareholders Meeting or any adjournment thereof, the EOP
Shareholder Approvals shall not have been obtained as contemplated by Section
5.1 or if the EOP Partner Approvals have not been obtained as contemplated by
Section 5.1;

     (h) by Cornerstone (i) if the Board of Directors of Cornerstone shall have
withdrawn, modified, amended or qualified in any manner adverse to EOP its
approval or recommendation of either of the Merger or this Agreement in
connection with, or approved or recommended, any Superior Acquisition Proposal,
or, (ii) in order to enter into a binding written agreement with respect to a
Superior Acquisition Proposal, provided that, in each case, Cornerstone shall
have complied with the terms of Section 4.3 and, prior to terminating pursuant
to this Section 7.1(h), has paid to EOP Partnership the Break-Up Fee (as defined
herein) as provided by Section 7.2 hereof; and

     (i) by EOP, if (1) the Board of Directors of Cornerstone shall have failed
to recommend or withdrawn, modified, amended or qualified, or proposed publicly
not to recommend or to withdraw, modify, amend or qualify, in any manner adverse
to EOP its approval or recommendation of either of the Mergers or this Agreement
or approved or recommended any Superior Acquisition Proposal, (2) following the
announcement or receipt of an Acquisition Proposal, Cornerstone shall have
failed to call the Cornerstone Stockholders Meeting in accordance with Section
5.1(a) or failed to prepare and mail to its stockholders

                                      A-53
<PAGE>   173

the Joint Proxy Statement in accordance with Section 5.1(a) or 5.1(b), or (3)
the Board of Directors of Cornerstone or any committee thereof shall have
resolved to do any of the foregoing.

     7.2 Certain Fees and Expenses. If this Agreement shall be terminated (i)
pursuant to Section 7.1(h), 7.1(i)(1) or 7.1(i)(3), then Cornerstone and
Cornerstone Partnership thereupon shall pay to EOP Partnership a fee equal to
the Break-Up Fee (as defined herein), and (ii) pursuant to Section 7.1(b) or
7.1(f), then Cornerstone and Cornerstone Partnership shall pay to EOP
Partnership (provided that Cornerstone was not entitled to terminate this
Agreement pursuant to Section 7.1(c) at the time of such termination) an amount
equal to the Break-Up Expenses (as defined herein). If this Agreement shall be
terminated pursuant to Section 7.1(c) or 7.1(g), then EOP and EOP Partnership
shall pay to Cornerstone Partnership (provided that EOP was not entitled to
terminate this Agreement pursuant to Section 7.1(b) at the time of such
termination) an amount equal to the Break-Up Expenses. If this Agreement shall
be terminated pursuant to Section 7.1(b), 7.1(d) (if primarily resulting from
any action or inaction of Cornerstone, Cornerstone Partnership or any
Cornerstone Subsidiary or the Cornerstone Non-controlled Subsidiary), 7.1(e),
7.1(f), 7.1(i)(2) and prior to the time of such termination an Acquisition
Proposal has been received by Cornerstone or Cornerstone Partnership, and either
prior to the termination of this Agreement or within twelve (12) months
thereafter, Cornerstone or Cornerstone Partnership enters into any written
agreement to consummate a transaction or series of transactions which, had such
agreement been proposed or negotiated during the term of this Agreement, would
have constituted an Acquisition Proposal pursuant to Section 4.3 (each, a
"Cornerstone Acquisition Agreement"), which is subsequently consummated (whether
or not any Cornerstone Acquisition Agreement relates to the same Acquisition
Proposal which had been received at the time of the termination of this
Agreement), then Cornerstone and Cornerstone Partnership shall pay the Break-Up
Fee to EOP Partnership.

     The payment of the Break-Up Fee shall be compensation for the loss suffered
by EOP and EOP Partnership as a result of the failure of the Mergers to be
consummated (including, without limitation, opportunity costs and out-of-pocket
costs and expenses) and to avoid the difficulty of determining damages under the
circumstances. The Break-Up Fee shall be paid by Cornerstone and Cornerstone
Partnership to EOP Partnership, or the Break-Up Expenses shall be paid by
Cornerstone and Cornerstone Partnership to EOP Partnership or EOP Partnership to
Cornerstone Partnership (as applicable), in immediately available funds within
two (2) business days after the date the event giving rise to the obligation to
make such payment occurred (except as otherwise provided in Section 7.1(h) or
7.1(i)). Cornerstone acknowledges that the agreements contained in this Section
7.2 are integral parts of this Agreement; accordingly, if Cornerstone and
Cornerstone Partnership fail to promptly pay the Break-Up Fee or Break-Up
Expenses due pursuant to this Section 7.2 and, in order to obtain payment, EOP
commences a suit which results in a judgment against Cornerstone or Cornerstone
Partnership for any amounts owed pursuant to this Section 7.2, Cornerstone and
Cornerstone Partnership shall pay to EOP its costs and expenses (including
attorneys' fees and expenses) in connection with such suit, together with
interest on the amount owed at the rate on six-month U.S. Treasury obligations
in effect on the date such payment was required to be made plus 300 basis
points.

     As used in this Agreement, "Break-Up Fee" shall be an amount equal to the
lesser of (i) $100,000,000 less Break-Up Expenses paid or payable under this
Section 7.2 (the "Base Amount") and (ii) the sum of (A) the maximum amount that
can be paid to EOP Partnership without causing EOP to fail to meet the
requirements of Sections 856(c)(2) and (3) of the Code determined as if the
payment of such amount did not constitute income described in Sections
856(c)(2)(A)-(H) and 856(c)(3)(A)-(I) of the Code ("Qualifying Income"), as
determined by independent accountants to EOP, and (B) in the event EOP receives
a letter from outside counsel (the "Break-Up Fee Tax Opinion") indicating that
EOP has received a ruling from the IRS holding that EOP Partnership's receipt of
the Base Amount would either constitute Qualifying Income or would be excluded
from gross income of EOP within the meaning of Sections 856(c)(2) and (3) of the
Code (the "REIT Requirements") or that the receipt by EOP Partnership of the
remaining balance of the Base Amount following the receipt of and pursuant to
such ruling would not be deemed constructively received prior thereto, the Base
Amount less the amount payable under clause (A) above. Cornerstone's and
Cornerstone Partnership's obligation to pay any unpaid

                                      A-54
<PAGE>   174

portion of the Break-Up Fee shall terminate three years from the date of this
Agreement. In the event that EOP Partnership is not able to receive the full
Base Amount, Cornerstone and Cornerstone Partnership shall place the unpaid
amount in escrow and shall not release any portion thereof to EOP Partnership
unless and until Cornerstone receives either one of the following: (i) a letter
from EOP's independent accountants indicating the maximum amount that can be
paid at that time to EOP Partnership without causing EOP to fail to meet the
REIT Requirements or (ii) a Break-Up Fee Tax Opinion, in either of which events
Cornerstone and Cornerstone Partnership shall pay to EOP Partnership the lesser
of the unpaid Base Amount or the maximum amount stated in the letter referred to
in (i) above.

     The "Break-Up Expenses" payable to EOP Partnership or Cornerstone
Partnership, as the case may be (the "Recipient"), shall be an amount equal to
the lesser of (i) $7,500,000 or (ii) the Recipient's out-of-pocket expenses
incurred in connection with this Agreement and the transactions contemplated
hereby (including, without limitation, all attorneys', accountants' and
investment bankers' fees and expenses). If the Break-Up Expenses payable to the
Recipient exceed the maximum amount that can be paid to the Recipient without
causing the Recipient to fail to meet the requirements of Sections 856(c)(2) and
(3) of the Code determined as if the payment of such amount did not constitute
Qualifying Income, as determined by independent accountants to the Recipient
(the "Maximum Amount"), the amount initially payable to the Recipient shall be
limited to the Maximum Amount. If, however, within the three-year period
commencing on the date of this Agreement, the Recipient receives a Break-Up Fee
Tax Opinion indicating that it has received a ruling from the IRS holding that
the Recipient's receipt of the Break-Up Expenses would either constitute
Qualifying Income or would be excluded from gross income of the Recipient within
the meaning of the REIT Requirements or that receipt by the Recipient of the
balance of the Break-Up Expenses above the Maximum Amount following the receipt
of and pursuant to such ruling would not be deemed constructively received prior
thereto, the Recipient shall be entitled to have payable to it the full amount
of the Break-Up Expenses. The obligation of EOP and EOP Partnership or
Cornerstone and Cornerstone Partnership, as applicable ("Payor"), to pay any
unpaid portion of the Break-Up Expenses shall terminate three years from the
date of this Agreement. In the event that the Recipient is not able to receive
the full Break-Up Expenses, the Payor shall place the unpaid amount in escrow
and shall not release any portion thereof to the Recipient unless and until the
Payor receives either one of the following: (i) a letter from the independent
accountants of EOP or Cornerstone, as the case may be, indicating the maximum
amount that can be paid at that time to the Recipient without causing it to fail
to meet the REIT Requirements or (ii) a Break-Up Expense Tax Opinion, in either
of which events the Payor shall pay to the Recipient the lesser of the unpaid
Break-Up Expenses or the maximum amount stated in the letter referred to in (i)
above.

     7.3 Effect of Termination. In the event of termination of this Agreement by
either Cornerstone or EOP as provided in Section 7.1, this Agreement shall
forthwith become void and have no effect, without any liability or obligation on
the part of EOP, EOP Partnership, Cornerstone or Cornerstone Partnership, other
than the last sentence of Section 5.2, Section 7.2, this Section 7.3 and Article
8, and except to the extent that such termination results from a material breach
by any party of any of its representations, warranties, covenants or agreements
set forth in this Agreement.

     7.4 Amendment. This Agreement may be amended by the parties in writing by
action of the respective Board of Trustees or Board of Directors of EOP and
Cornerstone at any time before or after any Shareholder Approvals are obtained
and prior to the filing of the Articles of Merger with the Department; provided,
however, that, after the Shareholder Approvals and Partner Approvals are
obtained, no such amendment, modification or supplement shall be made which by
law requires the further approval of shareholders or partners without obtaining
such further approval. The parties agree to amend this Agreement in the manner
provided in the immediately preceding sentence to the extent required to (a)
continue the status of each party as a REIT or (b) preserve the Merger as a
reorganization under Section 368(a) of the Code.

     7.5 Extension; Waiver. At any time prior to the Effective Time, the parties
may (a) extend the time for the performance of any of the obligations or other
acts of the other party, (b) waive any inaccuracies
                                      A-55
<PAGE>   175

in the representations and warranties of the other party contained in this
Agreement or in any document delivered pursuant to this Agreement or (c) subject
to the proviso of Section 7.4, waive compliance with any of the agreements or
conditions of the other party contained in this Agreement. Any agreement on the
part of a party to any such extension or waiver shall be valid only if set forth
in an instrument in writing signed on behalf of such party. The failure of any
party to this Agreement to assert any of its rights under this Agreement or
otherwise shall not constitute a waiver of those rights.

                                   ARTICLE 8

                               GENERAL PROVISIONS

     8.1 Nonsurvival of Representations and Warranties. None of the
representations and warranties in this Agreement or in any instrument delivered
pursuant to this Agreement confirming the representations and warranties in this
Agreement shall survive the Effective Time. This Section 8.1 shall not limit any
covenant or agreement of the parties which by its terms contemplates performance
after the Effective Time.

     8.2 Notices. All notices, requests, claims, demands and other
communications under this Agreement shall be in writing and shall be delivered
personally, sent by overnight courier (providing proof of delivery) to the
parties or sent by telecopy (providing confirmation of transmission) at the
following addresses or telecopy numbers (or at such other address or telecopy
number for a party as shall be specified by like notice):

     (a) if to EOP or EOP Partnership, to:

        Equity Office Properties Trust
        Two North Riverside Plaza, 22nd Floor
        Chicago, Illinois 60606
        Attention: Timothy H. Callahan, President
                Stanley M. Stevens, Chief Counsel
        Fax No.: (312) 559-5021

     with a copy to:

        Hogan & Hartson L.L.P.
        555 Thirteenth Street, N.W.
        Washington, D.C. 20004-1109
        Attention: J. Warren Gorrell, Jr., Esq.
                George P. Barsness, Esq.
        Fax No.: (202) 637-5910

     (b) if to Cornerstone or Cornerstone Partnership, to:

        Cornerstone Properties Inc.
        Tower 56
        126 East 56th Street, 6th Floor
        New York, New York 10022
        Attention: John S. Moody, President
        Fax No.: (212) 605-7199

                                      A-56
<PAGE>   176

     with a copy to:

        King & Spalding
        191 Peachtree Street
        Atlanta, GA 30303-1763
        Attention: William B. Fryer, Esq.
        Fax No.: (404) 572-5100

     All notices shall be deemed given only when actually received.

     8.3 Interpretation. When a reference is made in this Agreement to a
Section, such reference shall be to a Section of this Agreement unless otherwise
indicated. The table of contents and headings contained in this Agreement are
for reference purposes only and shall not affect in any way the meaning or
interpretation of this Agreement. Whenever the words "include", "includes" or
"including" are used in this Agreement, they shall be deemed to be followed by
the words "without limitation."

     8.4 Counterparts. This Agreement may be executed in one or more
counterparts, all of which shall be considered one and the same agreement and
shall become effective when one or more counterparts have been signed by each of
the parties and delivered to the other party.

     8.5 Entire Agreement; No Third-Party Beneficiaries. This Agreement, the
Cornerstone Disclosure Letter, the EOP Disclosure Letter, the Confidentiality
Agreement, the Voting Agreements and the other agreements entered into in
connection with the Mergers (a) constitute the entire agreement and supersede
all prior agreements and understandings, both written and oral between the
parties with respect to the subject matter of this Agreement and (b) except as
provided in Section 5.9 ("Third Party Provisions"), are not intended to confer
upon any person other than the parties hereto any rights or remedies.

     8.6 Governing Law. THE PARTNERSHIP MERGER SHALL BE GOVERNED BY, AND
CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF DELAWARE, REGARDLESS OF
THE LAWS THAT MIGHT OTHERWISE GOVERN UNDER APPLICABLE PRINCIPLES OF CONFLICT OF
LAWS THEREOF. EXCEPT AS PROVIDED IN THE IMMEDIATELY PRECEDING SENTENCE, THIS
AGREEMENT SHALL BE GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH, THE LAWS OF
THE STATE OF MARYLAND, REGARDLESS OF THE LAWS THAT MIGHT OTHERWISE GOVERN UNDER
APPLICABLE PRINCIPLES OF CONFLICT OF LAWS THEREOF.

     8.7 Assignment. Neither this Agreement nor any of the rights, interests or
obligations under this Agreement shall be assigned or delegated, in whole or in
part, by operation of law or otherwise by any of the parties without the prior
written consent of the other parties. Subject to the preceding sentence, this
Agreement will be binding upon, inure to the benefit of, and be enforceable by,
the parties and their respective successors and assigns.

     8.8 Enforcement. The parties agree that irreparable damage would occur in
the event that any of the provisions of this Agreement were not performed in
accordance with their specific terms or were otherwise breached. It is
accordingly agreed that the parties shall be entitled to an injunction or
injunctions to prevent breaches of this Agreement and to enforce specifically
the terms and provisions of this Agreement in any federal court located in
Maryland or in any state court located in Maryland this being in addition to any
other remedy to which they are entitled at law or in equity. In addition, each
of the parties hereto (a) consents to submit itself (without making such
submission exclusive) to the personal jurisdiction of any federal court located
in Maryland or any state court located in Maryland in the event any dispute
arises out of this Agreement or any of the transactions contemplated by this
Agreement and (b) agrees that it will not attempt to deny or defeat such
personal jurisdiction by motion or other request for leave from any such court.

     8.9 Severability. Any term or provision of this Agreement which is invalid
or unenforceable in any jurisdiction shall, as to that jurisdiction, be
ineffective to the extent of such invalidity or unenforceability without
rendering invalid or unenforceable the remaining terms and provisions of this
Agreement or

                                      A-57
<PAGE>   177

affecting the validity or enforceability of any of the terms or provisions of
this Agreement in any other jurisdiction. If any provision of this Agreement is
so broad as to be unenforceable, the provision shall be interpreted to be only
so broad as is enforceable.

     8.10 Exculpation. This Agreement shall not impose any personal liability on
any shareholder, trustee, trust manager, officer, employee or agent of EOP or
Cornerstone, and all Persons shall look solely to the property of EOP or
Cornerstone for the payment of any claim hereunder or for the performance of
this Agreement.

     8.11 Joint and Several Obligations. In each case where both Cornerstone and
Cornerstone Partnership, on the one hand, or EOP and EOP Partnership, on the
other hand, are obligated to perform the same obligation hereunder, such
obligation shall be joint and several.

                                      A-58
<PAGE>   178

     IN WITNESS WHEREOF, EOP, EOP Partnership, Cornerstone and Cornerstone
Partnership have caused this Agreement to be signed by their respective officers
(or general partners) thereunto duly authorized all as of the date first written
above.

                                            EQUITY OFFICE PROPERTIES TRUST

                                            By:   /s/ TIMOTHY H. CALLAHAN
                                              ----------------------------------
                                                Name: Timothy H. Callahan
                                              Title: President

                                            EOP OPERATING LIMITED PARTNERSHIP

                                            By: Equity Office Properties Trust,
                                            its
                                              managing general partner

                                            By:   /s/ TIMOTHY H. CALLAHAN
                                              ----------------------------------
                                                Name: Timothy H. Callahan
                                              Title: President

                                            CORNERSTONE PROPERTIES INC.

                                            By:      /s/ JOHN S. MOODY
                                              ----------------------------------
                                                Name: John S. Moody
                                              Title: President & CEO

                                            CORNERSTONE PROPERTIES LIMITED
                                            PARTNERSHIP

                                            By: Cornerstone Properties Inc.,
                                              its sole general partner

                                            By:      /s/ JOHN S. MOODY
                                              ----------------------------------
                                                Name: John S. Moody
                                              Title: President & CEO

                                      A-59
<PAGE>   179

                                                                         ANNEX B

                          [LETTERHEAD OF J.P. MORGAN]

February 11, 2000

The Board of Trustees
Equity Office Properties Trust
Two N. Riverside Plaza
Chicago, IL 60606

Attention: Mr. Samuel Zell
        Chairman

Ladies and Gentlemen:

     You have requested our opinion as to the fairness, from a financial point
of view, to Equity Office Properties Trust ("EOP" or the "Company") of the
consideration proposed to be paid by the Company in connection with the proposed
merger (the "Merger") of Cornerstone Properties Inc. (the "Seller") with and
into the company. Pursuant to the Agreement and Plan of Merger, dated as of
February 11, 2000 (the "Agreement"), among the Company, the Seller, and their
operating partnership subsidiaries, the Seller will be merged with and into the
Company, with the Company as the survivor of such merger, and each share of
common stock, no par value, of the Seller will be converted into the right to
receive either $18.00 or 0.7009 common shares of beneficial interest, $.01 par
value, of the Company, or a combination of such cash and stock consideration
according to the election and proration provisions in the Agreement.

     In arriving at our opinion, we have reviewed (i) the Agreement; (ii)
certain publicly available information concerning the business of the Seller and
of certain other companies engaged in businesses comparable to those of the
Seller, and the reported market prices for certain other companies' securities
deemed comparable; (iii) publicly available terms of certain transactions
involving companies comparable to the Seller and the consideration received for
such companies; (iv) current and historical market prices of the common stock of
the Seller and the Company; (v) the audited financial statements of the Company
and the Seller for the fiscal year ended December 31, 1998, and the unaudited
financial statements of the Company and the Seller for the period ended
September 30, 1999; (vi) certain agreements with respect to outstanding
indebtedness or obligations of the Company and the Seller; (vii) certain
internal financial analyses and forecasts prepared by the Company and the Seller
and their respective managements; and (viii) the terms of other business
combinations that we deemed relevant.

     In addition, we have held discussions with certain members of the
management of the Company and the Seller with respect to certain aspects of the
Merger, and the past and current business operations of the Company and the
Seller, the financial condition and future prospects and operations of the
Company and the Seller, the effects of the Merger on the financial condition and
future prospects of the Company and the Seller, and certain other matters we
believed necessary or appropriate to our inquiry. We have reviewed such other
financial studies and analyses and considered such other information as we
deemed appropriate for the purposes of this opinion.

     In giving our opinion, we have relied upon and assumed, without independent
verification, the accuracy and completeness in all material respects of all
information that was publicly available or was furnished to us by the Company
and the Seller or otherwise reviewed by us, and we have not assumed any
responsibility or liability therefor. We have not conducted any valuation or
appraisal of any assets or liabilities, nor have any such valuations or
appraisals been provided to us. In relying on financial analyses and forecasts
provided to us, we have assumed that they have been reasonably prepared based on
assumptions reflecting the best currently available estimates and judgments by
management as to the expected future results of operations and financial
condition of the Company and the Seller to which such analyses or forecasts
relate. We have also assumed that the Merger will have the tax consequences

                                       B-1
<PAGE>   180

described in discussions with, and materials furnished to us by, representatives
of the Company, and that the other transactions contemplated by the Agreement
will be consummated as described in the Agreement. We have relied as to all
legal matters relevant to rendering our opinion upon the advice of counsel.

     Our opinion is necessarily based on economic, market and other conditions
as in effect on, and the information made available to us as of, the date
hereof. It should be understood that subsequent developments may affect this
opinion and that we do not have any obligation to update, revise, or reaffirm
this opinion. We are expressing no opinion herein as to the price at which the
Company's shares will trade at any future time.

     We have acted as financial advisor to the Company with respect to the
proposed Merger and will receive a fee from the Company for our services
performed to date. We will also receive an additional fee if the proposed Merger
is consummated. In the ordinary course of their businesses, our affiliates may
actively trade the debt and equity securities of the Company or the Seller for
their own account or for the accounts of customers and, accordingly, they may at
any time hold long or short positions in such securities.

     On the basis of and subject to the foregoing, it is our opinion as of the
date hereof that the consideration proposed to be paid by the Company in the
Merger is fair, from a financial point of view, to the Company.

     This letter is provided to the Board of Trustees of the Company in
connection with and for the purposes of its evaluation of the Merger. This
opinion does not constitute a recommendation to any shareholder of the Company
as to how such shareholder should vote with respect to the Merger. This opinion
may not be disclosed, referred to, or communicated (in whole or in part) to any
third party for any purpose whatsoever except with our prior written consent in
each instance. This opinion may be reproduced in full in any filing made by the
Company with the Securities and Exchange Commission in connection with the
Merger.

Very truly yours,
J.P. MORGAN SECURITIES INC.

By:       /s/ JOHN PERKINS
    --------------------------------
    Name: John Perkins
    Title: Vice President

                                       B-2
<PAGE>   181

                                                                         ANNEX C

                         [LETTERHEAD OF LAZARD FRERES]

                                                      February 11, 2000

The Board of Directors
Cornerstone Properties Inc.
126 East 56th Street
New York, NY

Dear Members of the Board:

     We understand that Cornerstone Properties Inc. ("Cornerstone") has entered
into an Agreement and Plan of Merger with Equity Office Properties Trust
("Equity Office") dated as of February 11, 2000 (the "Merger Agreement"),
pursuant to which Cornerstone will merge with and into Equity Office and
Cornerstone Properties Limited Partnership ("Cornerstone OP") will merge with
and into EOP Operating Limited Partnership ("Equity Office OP") (collectively,
the "Merger"). Pursuant to the Merger (as set out in the Merger Agreement), each
outstanding share of common stock of Cornerstone ("Common Stock") would be
converted into the right to receive either $18.00 in cash or .7009 shares of
Equity Office common stock as elected by the holders of Cornerstone Common
Stock, and subject to pro ration and adjustment as provided in the Merger
Agreement.

     You have requested our opinion as to the fairness, from a financial point
of view, to the holders of Common Stock of the consideration to be received by
such holders in the Merger. In connection with this opinion, we have:

          (i) Reviewed the financial terms and conditions of the Merger
     Agreement;

          (ii) Analyzed certain historical business and financial information
     relating to Cornerstone and Equity Office;

          (iii) Reviewed various financial forecasts and other data provided to
     us by each of Cornerstone and Equity Office and relating to their
     respective businesses;

          (iv) Held discussions with members of the senior management of
     Cornerstone and Equity Office with respect to the businesses, prospects and
     strategic objectives of each company;

          (v) Reviewed public information with respect to certain other
     companies in lines of business we believe to be generally comparable to the
     business of Cornerstone;

          (vi) Reviewed the financial terms of certain business combinations
     involving companies in lines of business we believe to be generally
     comparable to those of Cornerstone;

          (vii) Reviewed the historical stock prices and trading volumes of the
     Common Stock and the common stock of Equity Office; and

          (viii) Conducted such other financial studies, analyses and
     investigations as we deemed appropriate.

     We have relied upon the accuracy and completeness of the foregoing
information, and have not assumed any responsibility for any independent
verification of such information or any independent valuation or appraisal of
any of the assets or liabilities of Cornerstone or Equity Office, or concerning
the solvency or fair value of Cornerstone, Cornerstone OP, Equity Office, Equity
Office OP or any other entity. With respect to financial forecasts, we have
assumed that they have been reasonably prepared on bases reflecting the best
currently available estimates and judgments of management of Cornerstone and
Equity Office as to the future financial performance of Cornerstone and Equity
Office, respectively. We

                                       C-1
<PAGE>   182

assume no responsibility for and express no view as to such forecasts or the
assumptions on which they are based.

     Further, our opinion is necessarily based on economic, monetary, market and
other conditions as in effect on, and the information made available to us as of
the date hereof. As you know, we have not been asked to solicit nor have we
solicited indications of interest from any other parties.

     In rendering our opinion, we have assumed that the Merger will be
consummated on the terms described in the Merger Agreement, without any waiver
of any material terms or conditions by Cornerstone or Cornerstone OP, and that
obtaining the necessary regulatory approvals for the Merger will not have an
adverse effect on Cornerstone or Cornerstone OP. We note that Cornerstone OP
unitholders have an option, exercisable prior to the Merger, to receive Equity
Office OP Units to be issued in the partnership merger or to redeem their units
for shares of Cornerstone Common Stock and elect to receive cash in the Merger
of Cornerstone into Equity Office, with respect to which we do not express a
view. However, our analysis has assumed that prior to the Merger each holder of
units in Cornerstone OP has elected to exercise its redemption right and receive
shares of Common Stock in Cornerstone in exchange therefor in the manner
provided in the Merger Agreement.

     Lazard Freres & Co. LLC is acting as investment banker to Cornerstone in
connection with the Merger and will receive a fee for our services paid partly
upon announcement and the balance upon the closing of the Merger. We have in the
past provided investment banking services to Cornerstone for which we have
received customary fees.

     Our engagement and the opinion expressed herein are for the benefit of
Cornerstone's Board of Directors and our opinion does not constitute a
recommendation as to how the holders of Common Stock of Cornerstone (and the
holders of limited partnership interests in Cornerstone OP) should vote with
respect to the matters contemplated by the Merger Agreement. Except as provided
in the engagement letter dated January 28, 2000, between Cornerstone and us, it
is understood that this letter may not be disclosed or otherwise referred to
without our prior consent, except as may otherwise be required by law or by a
court of competent jurisdiction.

     Based on and subject to the foregoing, we are of the opinion that the
consideration to be received in the Merger as set forth above is fair, from a
financial point of view, to the holders of Common Stock of Cornerstone.

                                            Very truly yours,

                                            LAZARD FRERES & CO. LLC

                                            By    /s/ MATTHEW J. LUSTIG
                                            ------------------------------------
                                                     Matthew J. Lustig
                                                     Managing Director

                                       C-2
<PAGE>   183

                                                                         ANNEX D

                AMENDMENTS TO EQUITY OFFICE DECLARATION OF TRUST
               RELATING TO "DOMESTICALLY-CONTROLLED" REIT STATUS

1. AMENDMENT TO SECTION 7.2.1(a)

     Section 7.2.1(a) is amended to add the following new subparagraph (iv):

          (iv) No Person shall acquire Beneficial Ownership of any Shares after
     the Effective Time if, as a result of such acquisition of Beneficial
     Ownership, the fair market value of the Shares owned directly and
     indirectly by Non-U.S. Persons for purposes of Section 897(h)(4)(B) of the
     Code would comprise forty three percent (43%) or more of the fair market
     value of the issued and outstanding Shares; provided, however, that the
     foregoing shall not apply to any acquisition of Beneficial Ownership of any
     Preferred Shares outstanding at the Effective Time or any Common Shares
     upon the conversion of any such Preferred Shares.

2. AMENDMENTS TO SECTION 7.1

     Section 7.1 is amended to add the following new definitions:

          Effective Time.  The term "Effective Time" shall mean the later of (i)
     the time the SDAT accepts for record articles of merger relating to the
     merger of Cornerstone Properties, Inc. with and into the Trust or (ii) the
     time established under such articles, not to exceed 30 days after the
     articles are accepted for record by the SDAT.

          Non-U.S. Person.  The term "Non-U.S. Person" shall mean a Person other
     than a U.S. Person.

          U.S. Person.  The term "U.S. Person" shall mean (a) a citizen or
     resident of the United States, (b) a partnership created or organized in
     the United States or under the laws of the United States or any state
     therein (including the District of Columbia), (c) a corporation created or
     organized in the United States or under the laws of the United States or
     any state therein (including the District of Columbia), and (d) any estate
     or trust (other than a foreign estate or foreign trust, within the meaning
     of Section 7701(a)(31) of the Code).

3. AMENDMENTS TO SECTION 7.2.1(b)

     Section 7.2.1(b) is amended to read as follows:

          (b) Transfer in Trust.  If any Transfer of Shares (whether or not such
     Transfer is the result of transaction entered into through the facilities
     of the NYSE or any other national securities exchange or automated
     inter-dealer quotation system) occurs which, if effective, would result in
     any Person Beneficially Owning or Constructively Owning Shares in violation
     of Section 7.2.1(a)(i), (ii) or (iv),

             (i) then that number of Shares the Beneficial or Constructive
        Ownership of which otherwise would cause such Person to violate Section
        7.2.1(a)(i), (ii) or (iv), as applicable (rounded up to the nearest
        whole share) shall be automatically transferred to a Charitable Trust
        for the benefit of a Charitable Beneficiary, as described in Section
        7.3, effective as of the close of business on the Business Day prior to
        the date of such Transfer, and such Person shall acquire no rights in
        such Shares; or

             (ii) if the transfer to the Charitable Trust described in clause
        (i) of this sentence would not be effective for any reason to prevent
        the violation of Section 7.2.1(a)(i), (ii) or (iv), as applicable, then
        the Transfer of that number of Shares that otherwise would cause such
        Person to violate Section 7.2.1(a)(i), (ii) or (iv), as applicable
        (rounded up to the nearest whole share) shall be void ab initio, and the
        intended transferee shall acquire no rights in such Shares.

                                       D-1
<PAGE>   184

4. AMENDMENT TO SECTION 7.2.4(b)

     The existing Section 7.2.4(b) is amended to read as follows:

          (b) each Person who is a Beneficial or Constructive Owner of Shares
     and each Person (including the shareholder of record) who is holding Shares
     for a Beneficial or Constructive Owner shall provide to the Trust such
     information as the Trust may require, in good faith, in order to determine
     the Trust's status as a REIT or a "domestically controlled REIT" (within
     the meaning of Section 897(h)(4)(B) of the Code) and to comply with the
     requirements of any taxing authority or to determine such compliance.

5. AMENDMENT TO ADD NEW SECTION 7.2.9

     The existing Section 7.2.9 is renumbered as Section 7.2.10 and the
following new Section 7.2.9 is added:

          Section 7.2.9 Increase in Percentage Set Forth in Section
     7.2.1(a)(iv).  The Board of Trustees may from time to time increase the
     percentage set forth in Section 7.2.1(a)(iv) from forty three percent (43%)
     to such higher percentage as shall be determined by the Board of Trustees;
     provided, however, that in no event shall such percentage exceed forty nine
     percent (49%) less the percentage of the aggregate fair market value of the
     total issued and outstanding Shares represented by the fair market value of
     any Preferred Shares then outstanding that were outstanding at the
     Effective Time (as such fair market values are determined by the Board of
     Trustees in good faith).

6. AMENDMENTS TO SECTION 7.2.10

     The first sentence of the legend set forth in Section 7.2.10 (as renumbered
pursuant to the other amendments made pursuant hereto) is amended to delete the
word "and" immediately preceding the following: "(iv) no Person may Transfer
Shares . . .", and to insert the following new language at the end of such first
sentence:

          ; and (v) no Person may acquire Beneficial Ownership of any Shares
     after the Effective Date if, as a result of such acquisition, the fair
     market value of the Shares owned directly and indirectly by Non-U.S.
     Persons would comprise more than forty three percent (43%) of the fair
     market value of the issued and outstanding Shares; provided, however, that
     clause (v) shall not apply to any acquisition of any Preferred Shares
     outstanding at the Effective Time.

                                       D-2
<PAGE>   185

                                    PART II

                     INFORMATION NOT REQUIRED IN PROSPECTUS

ITEM 20.  INDEMNIFICATION OF TRUSTEES AND OFFICERS

     Title 8 of the Corporations and Associations Article of the Annotated Code
of Maryland, as amended from time to time (the "Maryland REIT Law"), permits a
Maryland REIT to include in its declaration of trust a provision limiting the
liability of its trustees and officers to the trust and its shareholders for
money damages except for liability resulting from (a) actual receipt of an
improper benefit or profit in money, property or services or (b) active and
deliberate dishonesty established by a final judgment as being material to the
cause of action. The Equity Office declaration of trust contains such a
provision which eliminates such liability to the maximum extent permitted by the
Maryland REIT law.

     The Equity Office declaration of trust authorizes Equity Office, to the
maximum extent permitted by Maryland law, to obligate itself to indemnify and to
pay or reimburse reasonable expenses in advance of final disposition of a
proceeding to (a) any present or former trustee or officer or (b) any individual
who, while a trustee of Equity Office and at the request of Equity Office,
serves or has served as a director, officer, partner, trustee, employee or agent
of another corporation, partnership, joint venture, trust, employee benefit plan
or any other enterprise from and against any claim or liability to which such
person may become subject or which such person may incur by reason of his or her
status as a present or former trustee or officer of Equity Office. The Equity
Office bylaws obligate Equity Office, to the maximum extent permitted by
Maryland law, to indemnify and to pay or reimburse reasonable expenses in
advance of final disposition of a proceeding to (a) any present or former
trustee or officer who is made party to the proceeding by reason of his service
in that capacity or (b) any individual who, while a trustee or officer of Equity
Office and at the request of Equity Office, serves or has served another
corporation, partnership, joint venture, trust, employee benefit plan or any
other enterprise as a trustee, director, officer or partner of such corporation,
partnership, joint venture, trust, employee benefit plan or other enterprise and
who is made a party to the proceeding by reason of his service in that capacity,
against any claim or liability to which he or she may become subject by reason
of such status. The Equity Office declaration of trust and bylaws also permit
Equity Office to indemnify and advance expenses to any person who served a
predecessor of Equity Office in any of the capacities described above and to any
employee or agent of or a predecessor of Equity Office. The Equity Office bylaws
require Equity Office to indemnify a trustee or officer, or any former trustee
or officer, who has been successful, on the merits or otherwise, in the defense
of any proceeding to which he or she is made a party by reason of his service in
that capacity against reasonable expenses incurred in connection with the
proceeding.

     The Maryland REIT Law permits a Maryland REIT to indemnify and advance
expenses to its trustees, officers, employees and agents to the same extent as
permitted by the Maryland General Corporation Law, as amended from time to time
(the "MGCL"), for directors and officers of Maryland corporations. The MGCL
permits a corporation to indemnify its present and former directors and
officers, among others, against judgments, penalties, fines, settlements and
reasonable expenses actually incurred by them in connection with any proceeding
to which they may be made a party by reason of their service in those or other
capacities unless it is established that (a) the act or omission of the director
or officer was material to the matter giving rise to the proceeding and (x) was
committed in bad faith or (y) was the result of active and deliberate
dishonesty, (b) the director or officer actually received an improper personal
benefit in money, property or services or (c) in the case of any criminal
proceeding, the director or officer had reasonable cause to believe that the act
or omission was unlawful. The foregoing limitations on indemnification are
expressly set forth in the Equity Office bylaws. However, under the MGCL, a
Maryland corporation may not indemnify for any adverse judgment in a suit by or
in the right of the corporation or for a judgment of liability on the basis that
a personal benefit was improperly received, unless, in either case, a court
orders indemnification and then only for expenses. Under the MGCL, as a
condition to advancing expenses, as required by the Equity Office bylaws, Equity
Office must first receive (a) a written affirmation by the trustee or officer of
his good faith belief that he or she has met the

                                      II-1
<PAGE>   186

standard of conduct necessary for indemnification by Equity Office and (b) a
written undertaking by or on his behalf to repay the amount paid or reimbursed
by Equity Office if it shall ultimately be determined that the standard of
conduct was not met. In addition, Mr. Dobrowski, a trustee of Equity Office, is
indemnified by General Motors Investment Management Corporation and will be
covered by an insurance policy maintained by General Motors Corporation, of
which General Motors Investment Management Corporation is a subsidiary, in
connection with serving on the Equity Office board of trustees.

     The limited partnership agreement of EOP Operating Limited Partnership also
provides for indemnification of Equity Office and its officers and trustees to
the same extent that indemnification is provided to officers and trustees of
Equity Office in its declaration of trust, and limits the liability of Equity
Office and its officers and trustees to EOP Partnership and its respective
partners to the same extent that the Equity Office declaration of trust limits
the liability of the officers and trustees of Equity Office to Equity Office and
its shareholders.

     Insofar as indemnification for liabilities arising under the Securities Act
may be permitted to trustees, officers or persons controlling Equity Office
under the foregoing provisions, Equity Office has been informed that in the
opinion of the SEC such indemnification is against public policy as expressed in
the Act and is therefore unenforceable.

                                      II-2
<PAGE>   187

ITEM 21.  EXHIBITS AND FINANCIAL STATEMENT SCHEDULES

  (A) EXHIBITS

<TABLE>
<CAPTION>
EXHIBIT NO.                          EXHIBIT DESCRIPTION
- -----------                          -------------------
<C>              <S>
    2.1          Agreement and Plan of Merger, dated as of February 11, 2000,
                 among Equity Office, EOP Partnership, Cornerstone and
                 Cornerstone Partnership (included in this Registration
                 Statement as Annex A to the joint proxy
                 statement/prospectus)
    5.1*         Opinion of Hogan & Hartson L.L.P. regarding the legality of
                 common shares being registered
    8.1*         Opinion of Hogan & Hartson L.L.P. regarding certain federal
                 income tax consequences of the merger
    8.2*         Opinion of King & Spalding regarding certain federal income
                 tax consequences of the merger
   23.1          Consent of Ernst & Young, LLP (Equity Office)
   23.2          Consent of PricewaterhouseCoopers LLP (Cornerstone)
   23.3*         Consent of Hogan & Hartson L.L.P. (included in Exhibit 5.1)
   23.4*         Consent of Hogan & Hartson L.L.P. (included in Exhibits 8.1
                 and 8.3)
   23.5*         Consent of King & Spalding (included in Exhibits 8.2 and
                 8.4)
   23.6          Consent of J.P. Morgan Securities Inc.
   23.7          Consent of Lazard Freres & Co. LLC
   23.8          Consent of William Wilson III to be named as a trustee
   23.9          Consent of John S. Moody to be named as a trustee
   23.10         Consent of Jan H.W.R. van der Vlist to be named as a trustee
   24.1          Power of attorney (included on signature page)
   99.1          Stock Option Agreement, dated February 11, 2000, by and
                 among Cornerstone, Equity Office, Deutsche Bank AG and
                 Deutscher Herold Lebensversicherungs -- AG (incorporated by
                 reference to Exhibit 10.1 of Equity Office's Form 8-K dated
                 February 16, 2000)
   99.2          Voting Agreement, dated February 11, 2000, by and among
                 Equity Office, EOP Operating Limited Partnership, WCP
                 Services, Inc. and Stichting Pensioenfonds voor de
                 Gezonheid, Geestelijke en Maatschapelijke Belangen
                 (incorporated by reference to Exhibit 10.2 of Equity
                 Office's Form 8-K dated February 16, 2000)
   99.3*         Equity Office -- Form of Proxy
   99.4*         Cornerstone -- Form of Proxy
   99.5*         Form of Election
</TABLE>

- ---------------
* To be filed by amendment.

                                      II-3
<PAGE>   188

  (b) FINANCIAL STATEMENT SCHEDULES

     The following financial statement schedule was filed with the Equity Office
Annual Report on Form 10-K for the year ended December 31, 1999 (File no.
1-13115), filed with the SEC on March 28, 2000, and is incorporated herein by
reference:

  SCHEDULE III -- REAL ESTATE AND ACCUMULATED DEPRECIATION

     Schedules not listed above have been omitted because they are inapplicable
or the information required to be set forth therein is contained, or
incorporated by reference, in the consolidated final statements of Equity Office
or notes thereto.

ITEM 22.  UNDERTAKINGS

     The Registrant hereby undertakes:

          (1) To file, during any period in which offers or sales are being
     made, a post-effective amendment to this registration statement:

             (i) to include any prospectus required by section 10(a)(3) of the
        Securities Act of 1933;

             (ii) to reflect in the prospectus any facts or events arising after
        the effective date of the registration statement (or the most recent
        post-effective amendment thereof) which, individually or in the
        aggregate, represent a fundamental change in the information set forth
        in the registration statement. Notwithstanding the foregoing, any
        increase or decrease in volume of securities offered (if the total
        dollar value of securities offered would not exceed that which was
        registered) and any deviation from the low or high and of the estimated
        maximum offering range may be reflected in the form of prospectus filed
        with the Commission pursuant to Rule 424(b) if, in the aggregate, such
        changes in volume and price represent no more than a 20% change in the
        maximum aggregated offering price set forth in the "Calculation of
        Registration Fee" table in the effective registration statement;

             (iii) to include any material information with respect to the plan
        of distribution not previously disclosed in the registration statement
        or any material change to such information in the registration
        statement.

          (2) That, for the purpose of determining any liability under the
     Securities Act of 1933, each such post-effective amendment shall be deemed
     to be a new registration statement relating to the securities offered
     therein, and the offering of such securities at that time shall be deemed
     to be the initial bona fide offering thereof.

          (3) To remove from registration by means of a post-effective amendment
     any of the securities being registered which remain unsold at the
     termination of the offering.

          (4) That, for the purpose of determining any liability under the
     Securities Act of 1933, each filing of the registrant's annual report
     pursuant to section 13(a) or section 15(d) of the Securities Exchange Act
     of 1934 (and, where applicable, each filing of an employee benefit plan's
     annual report pursuant to section 15(d) of the Securities Exchange Act of
     1934) that is incorporated by reference in the registration statement shall
     be deemed to be a new registration statement relating to the securities
     offered therein, and the offering of such securities at that time shall be
     deemed to be the initial bona fide offering thereof.

          (5) That before any public reoffering of the securities registered
     hereunder through use of a prospectus which is a part of this registration
     statement, by any person or party who is deemed to be an underwriter within
     the meaning of Rule 145(c), the issuer undertakes that such reoffering
     prospectus will contain the information called for by the applicable
     registration form with respect to reofferings by persons who may be deemed
     underwriters, in addition to the information called for by the other items
     of the applicable form.

                                      II-4
<PAGE>   189

          (6) That every prospectus: (i) that is filed pursuant to paragraph (5)
     immediately preceding, or (ii) that purports to meet the requirements of
     Section 10(a)(3) of the Act and is used in connection with an offering of
     securities subject to Rule 415, will be filed as a part of an amendment to
     the registration statement and will not be used until such amendment is
     effective, and that, for purposes of determining any liability under the
     Securities Act of 1933, each such post-effective amendment shall be deemed
     to be a new registration statement relating to the securities offered
     therein, and the offering of such securities at that time shall be deemed
     to be the initial bona fide offering thereof.

          (7) Insofar as indemnification for liabilities arising under the
     Securities Act of 1933 may be permitted to trustees, officers and
     controlling persons of the registrant pursuant to the foregoing provisions,
     or otherwise, the registrant has been advised that in the opinion of the
     Securities and Exchange Commission such indemnification is against public
     policy as expressed in the Act and is, therefore, unenforceable. In the
     event that a claim for indemnification against such liabilities, other than
     the payment by the registrant of expenses incurred or paid by a trustee,
     officer or controlling person of the registrant in the successful defense
     of any action, suit or proceeding, is asserted by such trustee, officer or
     controlling person in connection with the securities being registered, the
     registrant will, unless in the opinion of its counsel the matter has been
     settled by controlling precedent, submit to a court of appropriate
     jurisdiction the question whether such indemnification by it is against
     public policy as expressed in the Act and will be governed by the final
     adjudication of such issue.

          (8) To respond to requests for information that is incorporated by
     reference into the prospectus pursuant to Item 4, 10(b), 11, or 13 of this
     form, within one business day of receipt of such request, and to send the
     incorporated documents by first class mail or other equally prompt means.
     This includes information contained in documents filed subsequent to the
     effective date of the registration statement through the date of responding
     to the request.

          (9) To supply by means of a post-effective amendment all information
     concerning a transaction, and the company being acquired involved therein,
     that was not the subject of and included in the registration statement when
     it became effective.

                                      II-5
<PAGE>   190

                                   SIGNATURES

     Pursuant to the requirements of the Securities Act, the registrant has duly
caused this Registration Statement to be signed on its behalf by the
undersigned, thereunto duly authorized, in the City of Chicago, State of
Illinois on March 29, 2000.

                                          EQUITY OFFICE PROPERTIES TRUST

                                          By:    /s/ TIMOTHY H. CALLAHAN
                                            ------------------------------------
                                              Timothy H. Callahan
                                            President and Chief Executive
                                              Officer

     We, the undersigned trustees and officers of Equity Office Properties
Trust, do hereby constitute and appoint Timothy H. Callahan and Stanley M.
Stevens, and each and either of them, our true and lawful attorneys-in-fact and
agents, to do any and all acts and things in our name and our behalf in our
capacities as trustees and officers and to execute any and all instruments for
us and in our name in the capacities indicated below, which said attorneys and
agents, or either of them, may deem necessary or advisable to enable said Trust
to comply with the Securities Act of 1933 and any rules, regulations and
requirements of the Securities and Exchange Commission, in connection with this
registration statement, including specifically, but without limitation, any and
all amendments, including post-effective amendments, hereto; and we hereby
ratify and confirm all that said attorneys and agents, or either of them, shall
do or cause to be done by virtue thereof.

     Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement has been signed by the following persons in the
capacities indicated as of the 29th day of March, 2000.

<TABLE>
<CAPTION>
                 SIGNATURE                                           TITLE
                 ---------                                           -----
<C>                                               <S>

          /s/ TIMOTHY H. CALLAHAN                 President, Chief Executive Officer and
- --------------------------------------------      Trustee (principal executive officer)
            TIMOTHY H. CALLAHAN

           /s/ RICHARD D. KINCAID                 Chief Financial Officer (principal financial
- --------------------------------------------      officer and principal accounting officer)
             RICHARD D. KINCAID

              /s/ SAMUEL ZELL                     Chairman of the Board of Trustees
- --------------------------------------------
                SAMUEL ZELL

           /s/ SHELI Z. ROSENBERG                 Trustee
- --------------------------------------------
             SHELI Z. ROSENBERG

          /s/ THOMAS E. DOBROWSKI                 Trustee
- --------------------------------------------
            THOMAS E. DOBROWSKI

          /s/ JAMES D. HARPER, JR.                Trustee
- --------------------------------------------
            JAMES D. HARPER, JR.

           /s/ JERRY M. REINSDORF                 Trustee
- --------------------------------------------
             JERRY M. REINSDORF

          /s/ William M. Goodyear                 Trustee
- --------------------------------------------
            WILLIAM M. GOODYEAR

            /s/ DAVID K. MCKOWN                   Trustee
- --------------------------------------------
              DAVID K. MCKOWN
</TABLE>

                                      II-6
<PAGE>   191

<TABLE>
<CAPTION>
             /s/ H. JON RUNSTAD                   Trustee
- --------------------------------------------
               H. JON RUNSTAD
<C>                                               <S>
            /s/ EDWIN N. SIDMAN                   Trustee
- --------------------------------------------
              EDWIN N. SIDMAN

          /s/ D. J. Andre de Bock                 Trustee
- --------------------------------------------
            D. J. ANDRE DE BOCK
</TABLE>

                                      II-7
<PAGE>   192

                                 EXHIBIT INDEX

<TABLE>
<CAPTION>
EXHIBIT NO.                          EXHIBIT DESCRIPTION
- -----------                          -------------------
<C>              <S>
    2.1          Agreement and Plan of Merger, dated as of February 11, 2000,
                 among Equity Office, EOP Partnership, Cornerstone and
                 Cornerstone Partnership (included in this Registration
                 Statement as Annex A to the joint proxy
                 statement/prospectus)
    5.1*         Opinion of Hogan & Hartson L.L.P. regarding the legality of
                 common shares being registered
    8.1*         Opinion of Hogan & Hartson L.L.P. regarding the
                 qualification of the merger as a reorganization for federal
                 income tax purposes and related federal income tax
                 consequences
    8.2*         Opinion of King & Spalding regarding the qualification of
                 the merger as a reorganization for federal income tax
                 purposes and related federal income tax consequences
    8.3*         Opinion of Hogan & Hartson L.L.P. regarding the
                 qualification of Equity Office Properties Trust as a real
                 estate investment trust for federal income tax purposes
    8.4*         Opinion of King & Spalding regarding the qualification of
                 Cornerstone Properties Inc. as a real estate investment
                 trust for federal income tax purposes
   23.1          Consent of Ernst & Young, LLP (Equity Office)
   23.2          Consent of PricewaterhouseCoopers LLP (Cornerstone)
   23.3*         Consent of Hogan & Hartson L.L.P. (included in Exhibit 5.1)
   23.4*         Consent of Hogan & Hartson L.L.P. (included in Exhibits 8.1
                 and 8.3)
   23.5*         Consent of King & Spalding (included in Exhibits 8.2 and
                 8.4)
   23.6          Consent of J.P. Morgan Securities Inc.
   23.7          Consent of Lazard Freres & Co. LLC
   23.8          Consent of William Wilson III to be named as a trustee
   23.9          Consent of John S. Moody to be named as a trustee
   23.10         Consent of Jan H.W.R. van der Vlist to be named as a trustee
   24.1          Power of attorney (included on signature page)
   99.1          Stock Option Agreement, dated February 11, 2000, by and
                 among Cornerstone, Equity Office, Deutsche Bank AG and
                 Deutscher Herold Lebensversicherungs -- AG (incorporated by
                 reference to Exhibit 10.1 of Equity Office's Form 8-K dated
                 February 16, 2000)
   99.2          Voting Agreement, dated February 11, 2000, by and among
                 Equity Office, EOP Operating Limited Partnership, WCP
                 Services, Inc. and Stichting Pensioenfonds voor de
                 Gezonheid, Geestelijke en Maatschapelijke Belangen
                 (incorporated by reference to Exhibit 10.2 of Equity
                 Office's Form 8-K dated February 16, 2000)
   99.3*         Equity Office -- Form of Proxy
   99.4*         Cornerstone -- Form of Proxy
   99.5*         Form of Election
</TABLE>

- ---------------
* To be filed by amendment.

                                      II-8

<PAGE>   1

                                                              Exhibit 23.1

We consent to the reference to our firm under the caption "Experts" in the
Registration Statement (Form S-4) and related Prospectus of Equity Office
Properties Trust for the registration of common shares of beneficial interest
related to the merger with Cornerstone Properties Inc. and to the incorporation
by reference of our report dated February 8, 2000, except for Note 24, as to
which the date is February 15, 2000 on the consolidated financial statements of
Equity Office Properties Trust included in its Annual Report (Form 10-K) for
the year ended December 31, 1999 filed with the Securities and Exchange
Commission.

                                                    /s/ Ernst & Young LLP
                                                    Ernst & Young LLP




Chicago, Illinois
March 27, 2000



<PAGE>   1
                                                                EXHIBIT 23.2


                       CONSENT OF INDEPENDENT ACCOUNTANTS

We hereby consent to the incorporation by reference in this Registration
Statement on Form S-4 of Equity Office Properties Trust of our report dated
March 3, 2000 relating to the financial statements and financial statement
schedule of Cornerstone Properties Inc., which appears in Cornerstone's Annual
Report on Form 10-K for the year ended December 31, 1999. We also consent to
the references to us under the headings "Experts" and "Selected Financial Data"
in such Registration Statement.


PricewaterhouseCoopers LLP


New York, New York
March 28, 2000



<PAGE>   1
                                                                EXHIBIT 23.6


[J.P. MORGAN LETTERHEAD]


                     CONSENT OF J.P. MORGAN SECURITIES INC.

         We hereby consent to (i) the use of our opinion letter dated February
11, 2000 to The Board of Trustees of Equity Office Properties Trust (the
"Company") included as Annex B to the Joint Proxy Statement/Prospectus which
forms a part of the Registration Statement on Form S-4 relating to the proposed
merger of the Company and Cornerstone Properties Inc., and (ii) the references
to such opinion in such Joint Proxy Statement/Prospectus. In giving such
consent, we do not admit that we come within the category of persons whose
consent is required under Section 7 of the Securities Act of 1933, as amended,
or the rules and regulations of the Securities and Exchange Commission
thereunder, nor do we hereby admit that we are experts with respect to any part
of such Registration Statement within the meaning of the term "experts" as used
in the Securities Act of 1933, as amended, or the rules and regulations of the
Securities and Exchange Commission thereunder.



J.P. MORGAN SECURITIES INC.

By:  /s/ John R. Perkins
     ------------------------------
     Name:  John R. Perkins
     Title:  Vice President

March 28, 2000



<PAGE>   1
                                                        EXHIBIT 23.7

                    [LETTERHEAD OF LAZARD FRERES & CO. LLC]

CONSENT OF LAZARD FRERES & CO. LLC

            We hereby consent to (i) the use of our opinion letter dated
February 11, 2000 to the Board of Directors of Cornerstone Properties Inc.
("Cornerstone"), included as Annex C to the Proxy Statement/Prospectus which
forms a part of the Registration Statement on Form S-4 relating to the proposed
acquisition of Cornerstone by Equity Office, and (ii) the references to such
opinion in such Proxy Statement/Prospectus. In giving such consent, we do not
admit that we come within the category or persons whose consent is required
under Section 7 of the Securities Act of 1933, as amended, or the rules and
regulations of the Securities and Exchange Commission thereunder, nor do we
hereby admit that we are "experts" as used in the Securities Act of 1933, as
amended, or the rules and regulations of the Securities and Exchange Commission
thereunder.



Dated:  March 28, 2000

                                        LAZARD FRERES & CO. LLC

                                        By:    /s/ Matthew J. Lustig
                                               -------------------------------
                                                 Name:  Matthew J. Lustig
                                                 Title: Managing Director



<PAGE>   1

                                                                   EXHIBIT 23.8

       CONSENT TO BE NAMED AS A TRUSTEE OF EQUITY OFFICE PROPERTIES TRUST

       I hereby consent, pursuant to Rule 438 of the Securities Act of 1933, as
amended, to be named as a person to become a trustee of Equity Office
Properties Trust, a Maryland real estate investment trust ("Equity Office"),
under the circumstances described in the registration statement on Form S-4
filed by Equity Office with the Securities and Exchange Commission in
connection with the merger of Cornerstone Properties Inc., a Nevada
corporation, with and into Equity Office.


                                                    /s/ William Wilson III
                                                    -------------------------
                                                    William Wilson III
                                                    Date: March 22, 2000




<PAGE>   1
                                                                EXHIBIT 23.9


       CONSENT TO BE NAMED AS A TRUSTEE OF EQUITY OFFICE PROPERTIES TRUST

         I hereby consent, pursuant to Rule 438 of the Securities Act of 1933,
as amended, to be named as a person to become a trustee of Equity Office
Properties Trust, a Maryland real estate investment trust ("Equity Office"),
under the circumstances described in the registration statement on Form S-4
filed by Equity Office with the Securities and Exchange Commission in
connection with the merger of Cornerstone Properties Inc., a Nevada
corporation, with and into Equity Office.

                                                   /s/ John S. Moody
                                                   --------------------------
                                                   John S. Moody
                                                   Date: March  22, 2000




<PAGE>   1
                                                                EXHIBIT 23.10


       CONSENT TO BE NAMED AS A TRUSTEE OF EQUITY OFFICE PROPERTIES TRUST

         I hereby consent, pursuant to Rule 438 of the Securities
Act of 1933, as amended, to be named as a person to become a trustee of Equity
Office Properties Trust, a Maryland real estate investment trust ("Equity
Office"), under the circumstances described in the registration statement on
Form S-4 filed by Equity Office with the Securities and Exchange Commission in
connection with the merger of Cornerstone Properties Inc., a Nevada
corporation, with and into Equity Office.

                                               /s/ Jan van der Vlist
                                               ----------------------------
                                               Jan van der Vlist
                                               Date: March 23, 2000




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