Treasury Regulations (26 C.F.R.)
26 CFR § 20.2056(d)-1 — (d)-1 Marital deduction; special rules for marital deduction if surviving spouse is not a United States citizen.
Rules pertaining to the application of section 2056(d), including certain transition rules, are contained in §§ 20.2056A-1 through 20.2056A-13.
Source: official text