26 USC 2010 - Unified credit against estate tax

(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 
For purposes of this section, the applicable credit amount is the amount of the tentative tax which would be determined under the rate schedule set forth in section 2001 (c) if the amount with respect to which such tentative tax is to be computed were the applicable exclusion amount determined in accordance with the following table: In the case of estates of decedents The applicable dying during: exclusion amount is: 2002 and 2003 $1,000,000 2004 and 2005 $1,500,000 2006, 2007, and 2008 $2,000,000 2009 $3,500,000.
(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.