26 U.S.C. § 2505

1. (a)In the case of a citizen or resident of the United States, there shall be allowed as a credit against the tax imposed by section 2501 for each calendar year an amount equal to—For purposes of applying paragraph (2) for any calendar year, the rates of tax in effect under section 2502(a)(2) for such calendar year shall, in lieu of the rates of tax in effect for preceding calendar periods, be used in determining the amounts allowable as a credit under this section for all preceding calendar periods.
   1. (1)the applicable credit amount in effect under section 2010(c) which would apply if the donor died as of the end of the calendar year, reduced by
   2. (2)the sum of the amounts allowable as a credit to the individual under this section for all preceding calendar periods.
2. (b)The amount 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 individual after September 8, 1976.
3. (c)The amount of the credit allowed under subsection (a) for any calendar year shall not exceed the amount of the tax imposed by section 2501 for such calendar year.