26 CFR § 25.2523(c)-1 - Interest in unidentified assets.
(a) Section 2523(c) provides that if an interest passing to a donee spouse may be satisfied out of a group of assets (or their proceeds) which include a particular asset that would be a nondeductible interest if it passed from the donor to his spouse, the value of the interest passing to the spouse is reduced, for the purpose of the marital deduction, by the value of the particular asset.
(b) In order for this section to apply, two circumstances must coexist, as follows:
(1) The property interest transferred to the donee spouse must be payable out of a group of assets. An example of a property interest payable out of a group of assets is a right to a share of the corpus of a trust upon its termination.
(2) The group of assets out of which the property interest is payable must include one or more particular assets which, if transferred by the donor to the donee spouse, would not qualify for the marital deduction. Therefore, section 2523 (c) is not applicable merely because a group of assets includes a terminable interest, but would only be applicable if the terminable interest were nondeductible under the provisions of § 25.2523(b)-1.
(c) If both of the circumstances set forth in paragraph (b) of this section exist, only a portion of the property interest passing to the spouse is a deductible interest. The portion qualifying as a deductible interest is an amount equal to the excess, if any, of the value of the property interest passing to the spouse over the aggregate value of the asset (or assets) that if transferred to the spouse would not qualify for the marital deduction. See paragraph (c) of § 25.2523(a)-l to determine the percentage of the deductible interest allowable as a marital deduction. The application of this section may be illustrated by the following example: