Generally, the federal law looks to state law to determine tax apportionment. Riggs v. Del Drago: “Congress from 1916 onward has understood local law as governing the distribution of the estate tax after payment of the tax.” Also “Congress did not contemplate that the Government would be interested in the distribution of the estate after the tax was paid, and that Congress intended that state law should determine the ultimate thrust of the tax.” Also: “If the issue is how to apportion the estate taxes, Riggs v. Del Drago instructs us to look to state law.” Estate of Reno v. Comm.
1.2.1 This does not mean that the IRS is restricted in its collection efforts to follow the tax apportionment scheme. Under IRC § 6324, for example, a “secret” estate tax lien attaches to all of a decedent’s probate property and the IRS is able to chase that property into the hands of as bone fide purchaser.
1.2.2 Although the general federal law refers to state law, certain provisions of federal law contain special apportionment provisions. IRC § 2603 (b) provides that “Unless otherwise directed pursuant to the governing instrument by specific reference to the tax imposed by this chapter, the tax imposed by this chapter on a generation-skipping transfer shall be charged to the property constituting such transfer.” Also, IRC § 2207 provides that unless the decedent provides otherwise, the property subject to a general power of appointment shall bear its share of the estate tax and, similarly, IRC § 2207A provides that QTIP property likewise bears its portion of the tax. Riggs v. Del Drago, 317 U.S. 95, at 99 and 98 (1944).
 Estate of Reno v. Comm., 945 F. 2d 733, 733 (4th Cir. 1991) (citation omitted)..
 Detroit Bank v. U.S., 317 U.S. 329 (1943); U.S. v. Vohland, 675 F2d 1071 (1982 CA9).