(a) In general. Property interests which passed from a decedent to his surviving spouse fall within two general categories:

(1) Those with respect to which the marital deduction is authorized, and

(2) Those with respect to which the marital deduction is not authorized.

(b) Deductible interests. An interest passing to a decedent's surviving spouse is a “deductible interest” if it does not fall within one of the following categories of “nondeductible interests”;

(1) Any property interest which passed from the decedent to his surviving spouse is a “nondeductible interest” to the extent it is not included in the decedent's gross estate.

(2) If a deduction is allowed under section 2053 (relating to deductions for expenses and indebtedness) by reason of the passing of a property interest from the decedent to his surviving spouse, such interest is, to the extent of the deduction under section 2053, a “nondeductible interest.” Thus, a property interest which passed from the decedent to his surviving spouse in satisfaction of a deductible claim of the spouse against the estate is, to the extent of the claim, a “nondeductible interest” (see § 20.2056(b)-4). Similarly, amounts deducted under section 2053(a)(2) for commissioners allowed to the surviving spouse as executor are “nondeductible interests”. As to the valuation, for the purpose of the marital deduction, of any property interest which passed from the decedent to his surviving spouse subject to a mortgage or other encumbrance, see § 20.2056(b)-4.

(3) If during settlement of the estate a loss deductible under section 2054 occurs with respect to a property interest, then that interest is, to the extent of the deductible loss, a “nondeductible interest” for the purpose of the marital deduction.

(4) A property interest passing to a decedent's surviving spouse which is a “terminable interest”, as defined in § 20.2056(b)-1, is a “nondeductible interest” to the extent specified in that section.