26 C.F.R. § 20.2055-6
Disallowance of double deduction in the case of qualified terminable interest property
No deduction is allowed from the decedent's gross estate under section 2055 for property with respect to which a deduction is allowed by reason of section 2056(b)(7). See section 2056(b)(9) and § 20.2056(b)-9.