A property interest is treated as passing to the surviving spouse only if it passes to the spouse as beneficial owner, except to the extent otherwise provided in §§ 20.2056(b)-5 through 20.2056(b)-7 . For this purpose, where a property interest passed from the decedent in trust, such interest is considered to have passed from him to his surviving spouse to the extent of her beneficial interest therein. The deduction may not be taken with respect to a property interest which passed to such spouse merely as trustee, or subject to a binding agreement by the spouse to dispose of the interest in favor of a third person. An allowance or award paid to a surviving spouse pursuant to local law for her support during the administration of the decedent's estate constitutes a property interest passing from the decedent to his surviving spouse. In determining whether or not such an interest is deductible, however, see generally the terminable interest rules of § 20.2056(b)-1 and especially example (8) of paragraph (g) of that section.
26 C.F.R. §20.2056(c)-2