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(a) General rule: A credit of the applicable credit amount shall be allowed to the estate of every decedent against the tax imposed by section 2001.

(b) Adjustment to credit for certain gifts made before 1977
The amount of the credit allowable under subsection (a) shall be reduced by an amount equal to 20 percent of the aggregate amount allowed as a specific exemption under section 2521 (as in effect before its repeal by the Tax Reform Act of 1976) with respect to gifts made by the decedent after September 8, 1976.

(c) Applicable credit amount
     (1) In general: For purposes of this section, the applicable credit amount is the amount of the tentative tax which would be determined under section 2001 (c) if the amount with respect to which such tentative tax is to be computed were equal to the applicable exclusion amount.
   
     (2) Applicable exclusion amount: For purposes of this subsection, the applicable exclusion amount is the sum of—
           (A) the basic exclusion amount, and
           (B) in the case of a surviving spouse, the deceased spousal unused exclusion amount.

     (3) Basic exclusion amount
           (A) In general: For purposes of this subsection, the basic exclusion amount is $5,000,000.
           (B) Inflation adjustment: In the case of any decedent dying in a calendar year after 2011, the dollar amount in subparagraph (A) shall be increased by an amount equal to—
                 (i) such dollar amount, multiplied by
                 (ii) the cost-of-living adjustment determined under section 1 (f)(3) for such calendar year by substituting “calendar year 2010” for “calendar year 1992” in subparagraph (B) thereof.
If any amount as adjusted under the preceding sentence is not a multiple of $10,000, such amount shall be rounded to the nearest multiple of $10,000.

     (4) Deceased spousal unused exclusion amount: For purposes of this subsection, with respect to a surviving spouse of a deceased spouse dying after December 31, 2010, the term “deceased spousal unused exclusion amount” means the lesser of—
          (A) the basic exclusion amount, or
          (B) the excess of—
                 (i) the applicable exclusion amount of the last such deceased spouse of such surviving spouse, over
                 (ii) the amount with respect to which the tentative tax is determined under section2001 (b)(1) on the estate of such deceased spouse.

     (5) Special rules
           (A) Election required: A deceased spousal unused exclusion amount may not be taken into account by a surviving spouse under paragraph (2) unless the executor of the estate of the deceased spouse files an estate tax return on which such amount is computed and makes an election on such return that such amount may be so taken into account. Such election, once made, shall be irrevocable. No election may be made under this subparagraph if such return is filed after the time prescribed by law (including extensions) for filing such return.
           (B) Examination of prior returns after expiration of period of limitations with respect to deceased spousal unused exclusion amount: Notwithstanding any period of limitation in section 6501, after the time has expired under section 6501 within which a tax may be assessed under chapter 11 or 12 with respect to a deceased spousal unused exclusion amount, the Secretary may examine a return of the deceased spouse to make determinations with respect to such amount for purposes of carrying out this subsection.

    (6) Regulations: The Secretary shall prescribe such regulations as may be necessary or appropriate to carry out this subsection.

(d) Limitation based on amount of tax
The amount of the credit allowed by subsection (a) shall not exceed the amount of the tax imposed by section 2001.