26 CFR § 20.2056(c)-1 - Marital deduction; definition of “passed from the decedent.”
(a) In general. The following rules are applicable in determining the person to whom any property interest “passed from the decedent”:
(1) Property interests devolving upon any person (or persons) as surviving coowner with the decedent under any form of joint ownership under which the right of survivorship existed are considered as having passed from the decedent to such person (or persons).
(2) Property interests at any time subject to the decedent's power to appoint (whether alone or in conjunction with any person) are considered as having passed from the decedent to the appointee under his exercise of the power, or, in case of the lapse, release or nonexercise of the power, as having passed from the decedent to the taker in default of exercise.
(3) The dower or curtesy interest (or statutory interest in lieu thereof) of the decedent's surviving spouse is considered as having passed from the decedent to his spouse.
(4) The proceeds of insurance upon the life of the decedent are considered as having passed from the decedent to the person who, at the time of the decedent's death, was entitled to receive the proceeds.
(5) Any property interest transferred during life, bequeathed or devised by the decedent, or inherited from the decedent, is considered as having passed to the person to whom he transferred, bequeathed, or devised the interest, or to the person who inherited the interest from him.
(6) The survivor's interest in an annuity or other payment described in section 2039 (see §§ 20.2039-1 and 20.2039-2) is considered as having passed from the decedent to the survivor only to the extent that the value of such interest is included in the decedent's gross estate under that section. If only a portion of the entire annuity or other payment is included in the decedent's gross estate and the annuity or other payment is payable to more than one beneficiary, then the value of the interest considered to have passed to each beneficiary is that portion of the amount payable to each beneficiary that the amount of the annuity or other payment included in the decedent's gross estate bears to the total value of the annuity or other payment payable to all beneficiaries.
(b) Expectant interest in property under community property laws. If before the decedent's death the decedent's surviving spouse had merely an expectant interest in property held by her and the decedent under community property laws, that interest is considered as having passed from the decedent to the spouse.