Exhibit 4.b

                                                             Executed in 6 Parts
                                                             Counterpart No. ( )

                              NATIONAL EQUITY TRUST

                          LOW FIVE PORTFOLIO SERIES 51

                            REFERENCE TRUST AGREEMENT

      This Reference Trust Agreement dated February 26, 2003 among Prudential
Investment Management Services LLC, as Depositor, Prudential Securities
Incorporated, as Portfolio Supervisor, and The Bank of New York, as Trustee,
sets forth certain provisions in full and incorporates other provisions by
reference to the document entitled "National Equity Trust, Trust Indenture and
Agreement" (the "Basic Agreement") dated February 2, 2000. Such provisions as
are set forth in full herein and such provisions as are incorporated by
reference constitute a single instrument (the "Indenture").

                                WITNESSETH THAT:

      In consideration of the premises and of the mutual agreements herein
contained, the Depositor and the Trustee agree as follows:

                                     Part I.

                     STANDARD TERMS AND CONDITIONS OF TRUST

    Subject to the provisions of Part II hereof, all the provisions contained
in the Basic  Agreement are herein  incorporated  by reference in their entirety
and  shall be deemed  to be a part of this  instrument  as fully and to the same
extent as though said provisions had been set forth in full in this instrument.

      A. Article I, entitled "Definitions," shall be amended as follows:

      (i) Section 1.01-Definitions shall be amended to add the following
definition at the end thereof:

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                                       -2-

      "Portfolio Supervisor" of the Trust shall have the meaning assigned to it
in Part II of the Reference Trust Agreement.

      B. Article III, entitled "Administration of Trust," shall be amended as
follows:

      (i) The third paragraph of Section 3.05-Distribution shall be amended by
deleting any reference to Depositor and replacing it with Portfolio Supervisor.

      (ii) Section 3.14-Deferred Sales Charge shall be amended to add the
following sentences at the end thereof:

      "References to Deferred Sales Charge in this Trust Indenture and Agreement
      shall include any Creation and Development Fee indicated in the prospectus
      for a Trust. The Creation and Development Fee shall be payable on each
      date so designated and in an amount determined as specified in the
      prospectus for a Trust."

      C. Article VIII, entitled "Depositor," shall be amended as follows:

      (i) Section 8.07-Compensation shall be amended by deleting any reference
to Depositor and replacing it with Portfolio Supervisor.

      D. Article IX, entitled "Additional Covenants; Miscellaneous Provisions,"
shall be amended as follows:

      (i) The first sentence of Section 9.05 - Written Notice shall be amended
by deleting the language "Prudential Securities Incorporated at One Seaport
Plaza, New York, New York 10292" and replacing it with "Prudential Investment
Management LLC at 100 Mulberry Street, Gateway Center Three, Newark, New Jersey
07102".

                                    Part II.

                      SPECIAL TERMS AND CONDITIONS OF TRUST

      The following special terms and conditions are hereby agreed to:
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                                       -3-

      A. The Trust is denominated National Equity Trust, Low Five Portfolio
Series 51.

      B. The Units of the Trust shall be subject to a deferred sales charge.

      C. The publicly traded stocks listed in Schedule A hereto are those which,
subject to the terms of this Indenture, have been or are to be deposited in
Trust under this Indenture as of the date hereof.

      D. The term "Depositor" shall mean Prudential Investment Management
Services LLC.

      E. The term "Portfolio Supervisor" shall mean Prudential Securities
Incorporated.

      F. The aggregate number of Units referred to in Sections 2.03 and 9.01 of
the Basic Agreement is 125,000 as of the date hereof.

      G. A Unit of the Trust is hereby declared initially equal to 1/125,000th
of the Trust.

      H. The term "First Settlement Date" shall mean March 4, 2003.

      I. The terms "Computation Day" and "Record Date" shall mean on the tenth
day of June 2003, September 2003, December 2003 and March 2004.

      J. The term "Distribution Date" shall mean on the twenty-fifth day of
June 2003, September 2003, December 2003 and March 2004.

      K. The term "Termination Date" shall mean April 7, 2004.

      L. The Trustee's Annual Fee shall be $.90 (per 1,000 Units) for 49,999,999
and below units outstanding $.84 (per 1,000 Units) on the next 50,000,000 Units,
$.78 (per 1,000 Units) on the next 100,000,000 Units, and $.66 (per 1,000 Units)
on Units in excess of 200,000,000 Units. In calculating the Trustee's annual
fee, the fee applicable to the number of units outstanding shall apply to all
units outstanding.

      M. The Portfolio Supervisor's portfolio supervisory service fee shall be
$.25 per 1,000 Units.

               [Signatures and acknowledgments on separate pages]
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                                       -4-

The Schedule of Portfolio Securities in Part A of the prospectus included in
this Registration Statement for National Equity Trust, Low Five Portfolio Series
51 is hereby incorporated by reference herein as Schedule A hereto.

<PAGE>

                                            PRUDENTIAL INVESTMENT MANAGEMENT
                                            SERVICES LLC,
                                            (Depositor)

                                            By /s/ Richard R. Hoffmann
                                               ---------------------------
                                                   Richard R. Hoffmann
                                                   Vice President

                                            PRUDENTIAL SECURITIES INCORPORATED
                                            (Portfolio Supervisor)

                                            By /s/ Richard R. Hoffmann
                                               ---------------------------
                                                   Richard R. Hoffmann
                                                   Vice President

<PAGE>

                                                       THE BANK OF NEW YORK
                                                       Trustee

                                            By: /s/ Brian Aarons
                                               ---------------------------
                                                    Brian Aarons
                                                    Title: Vice President

(SEAL)

ATTEST:

By: /s/ Dorothy Alencastro
        Dorothy Alencastro
        -------------------------------
        Title: Assistant Vice President

STATE OF NEW YORK          )
                             : ss.:
COUNTY OF NEW YORK         )

      I, Emanuel Lytle, Jr., a Notary Public in and for the said County in the
state aforesaid, do hereby certify that Brian Aarons and Dorothy Alencastro
personally known to me to be the same persons whose names are subscribed to the
foregoing instrument and personally known to me to be a Vice President and
Assistant Vice President, respectively of The Bank of New York, appeared before
me this day in person, and acknowledged that they signed, sealed with a
corporate seal of The Bank of New York, and delivered the said instrument as
their free and voluntary act as such Vice President and Assistant Vice
President, respectively, and as the free and voluntary act of The Bank of New
York, for the uses and purposes therein set forth.

      GIVEN, under my hand and notarial seal this 26th day of February, 2003.

                                               /s/ Emanuel Lytle, Jr.
                                               ---------------------------------
                                                   Emanuel Lytle, Jr.
                                                   Notary Public

(SEAL)