(a) In general. In the case of a trust that is subject to subpart C of part I of subchapter J of chapter 1 of the Code (relating to estates and trusts that may accumulate income or that distribute corpus), section 666(a) prescribes rules for determining the taxable years from which an accumulation distribution will be deemed to have been made and the extent to which the accumulation distribution is considered to consist of undistributed net income. In general, an accumulation distribution made in taxable years beginning after December 31, 1969, is deemed to have been made first from the earliest preceding taxable year of the trust for which there is undistributed net income. An accumulation distribution made in a taxable year beginning before January 1, 1970, is deemed to have been made first from the most recent preceding taxable year of the trust for which there is undistributed net income. See § 1.665(e)-1A for the definition of “preceding taxable year.”

(b) Distributions by domestic trusts—(1) Taxable years beginning after December 31, 1973. An accumulation distribution made by a trust (other than a foreign trust created by a U.S. person) in any taxable year beginning after December 31, 1973, is allocated to the preceding taxable years of the trust (defined in § 1.665(e)-1A(a)(1)(ii) as those beginning after December 31, 1968) according to the amount of undistributed net income of the trust for such years. For this purpose, an accumulation distribution is first to be allocated to the earliest such preceding taxable year in which there is undistributed net income and shall then be allocated, beginning with the next earliest, to any remaining preceding taxable years of the trust. The portion of the accumulation distribution allocated to the earliest preceding taxable year is the amount of the undistributed net income for that preceding taxable year. The portion of the accumulation distribution allocated to any preceding taxable year subsequent to the earliest such preceding taxable year is the excess of the accumulation distribution over the aggregate of the undistributed net income for all earlier preceding taxable years. See paragraph (d) of this section for adjustments to undistributed net income for prior distributions. The provisions of this subparagraph may be illustrated by the following example:

(2) Taxable years beginning after December 31, 1969, and before January 1, 1974. If a trust (other than a foreign trust created by a U.S. person) makes an accumulation distribution in a taxable year beginning after December 31, 1969, and before January 1, 1974, the distribution will be deemed distributed in the same manner as accumulation distributions qualifying under subparagraph (1) of this paragraph, except that the first year to which the distribution may be thrown back cannot be earlier than the fifth taxable year of the trust preceding the year in which the accumulation distribution is made. Thus, for example, in the case of an accumulation distribution made in the taxable year of a domestic trust which begins on January 1, 1972, the taxable year of the trust beginning on January 1, 1967, would be the first year in which the distribution was deemed made, assuming that there was undistributed net income for 1967. See also § 1.665(e)-1A(a)(1). The provisions of this subparagraph may be illustrated by the following example:

(3) Taxable years beginning after December 31, 1968, and before January 1, 1970. Accumulation distributions made in taxable years of the trust beginning after December 31, 1968, and before January 1, 1970, are allocated to prior years according to § 1.666(a)-1.

(c) Distributions by foreign trusts—(1) Foreign trusts created solely by U.S. persons—(i) Taxable years beginning after December 31, 1969. If a foreign trust created by a U.S. person makes an accumulation distribution in any taxable year beginning after December 31, 1969, the distribution is allocated to the trust's preceding taxable years (defined in § 1.665(e)-1A(a)(2) as those beginning after Dec. 31, 1953, and ending after Aug. 16, 1954) according to the amount of undistributed net income of the trust for such years. For this purpose, an accumulation distribution is first allocated to the earliest such preceding taxable year in which there is undistributed net income and shall then be allocated in turn, beginning with the next earliest, to any remaining preceding taxable years of the trust. The portion of the accumulation distribution allocated to the earliest preceding taxable year is the amount of the undistributed net income for that preceding taxable year. The portion of the accumulation distribution allocated to any preceding taxable year subsequent to the earliest such preceding taxable year is the excess of the accumulation distribution over the aggregate of the undistributed net income for all earlier preceding taxable years. See paragraph (d) of this section for adjustments to undistributed net income for prior distributions. The provisions of this subdivision may be illustrated by the following example:

(ii) Taxable years beginning after December 31, 1968, and before January 1, 1970. Accumulation distributions made in taxable years of the trust beginning after December 31, 1968, and before January 1, 1970, are allocated to prior years according to § 1.666(a)-1.

(2) Foreign trusts created partly by U.S. persons—(i) Taxable years beginning after December 31, 1969. If a trust that is in part a foreign trust created by a U.S. person and in part a foreign trust created by a person other than a U.S. person makes an accumulation distribution in any year after December 31, 1969, the distribution is deemed made from the undistributed net income of the foreign trust created by a U.S. person in the proportion that the total undistributed net income for all preceding years of the foreign trust created by the U.S. person bears to the total undistributed net income for all years of the entire foreign trust. In addition, such distribution is deemed made from the undistributed net income of the foreign trust created by a person other than a U.S. person in the proportion that the total undistributed net income for all preceding years of the foreign trust created by a person other than a U.S. person bears to the total undistributed net income for all years of the entire foreign trust. Accordingly, an accumulation distribution of such a trust is composed of two portions with one portion relating to the undistributed net income of the foreign trust created by the U.S. person and the other portion relating to the undistributed net income of the foreign trust created by the person other than a U.S. person. For these purposes, each portion of an accumulation distribution made in any taxable year is first allocated to each of such preceding taxable years in turn, beginning with the earliest preceding taxable year, as defined in § 1.665(e)-1A(a), of the applicable foreign trusts, to the extent of the undistributed net income for the such trust for each of those years. Thus, each portion of an accumulation distribution is deemed to have been made from the earliest accumulated income of the applicable trust. If the foreign trust created by a U.S. person makes an accumulation distribution in any year beginning after December 31, 1969, the distribution is included in the beneficiary's income for that year to the extent of the undistributed net income of the trust for the trust's preceding taxable years which began after December 31, 1953, and ended after August 16, 1954. The provisions of this subdivision may be illustrated by the following example:

(ii) Taxable years beginning after December 31, 1968, and before January 1, 1970. Accumulation distributions made in taxable years of the trust beginning after December 31, 1968, and before January 1, 1970, are allocated to prior years according to § 1.666(a)-1.

(3) Foreign trusts created by non-U.S. persons. To the extent that a foreign trust is a foreign trust created by a person other than a U.S. person, an accumulation distribution is included in the beneficiary's income for the year paid, credited, or required to be distributed to the extent provided under paragraph (b) of this section.

(d) Reduction of undistributed net income for prior accumulation distributions. For the purposes of allocating to any preceding taxable year an accumulation distribution of the taxable year, the undistributed net income of such preceding taxable year is reduced by the amount from such year deemed distributed in any accumulation distribution of undistributed net income made in any taxable year intervening between such preceding taxable year and the taxable year. Accordingly, for example, if a trust has undistributed net income for 1974 and makes accumulation distributions during the taxable years 1978 and 1979, in determining that part of the 1979 accumulation distribution that is thrown back to 1974 the undistributed net income for 1974 is first reduced by the amount of the undistributed net income for 1974 deemed distributed in the 1978 accumulation distribution.

(e) Rule when no undistributed net income. If, before the application of the provisions of subpart D to an accumulation distribution for the taxable year, there is no undistributed net income for a preceding taxable year, then no portion of the accumulation distribution is undistributed net income deemed distributed on the last day of such preceding taxable year. Thus, if an accumulation distribution is made during the taxable year 1975 from a trust whose earliest preceding taxable year is taxable year 1970, and the trust had no undistributed net income for 1970, then no portion of the 1975 accumulation distribution is undistributed net income deemed distributed on the last day of 1970.