26 C.F.R. § 1.665(a)-0
Subpart D (section 665 and following), part I, subchapter J, chapter 1 of the Internal Revenue Code, in the case of trusts other than foreign trusts created by U.S. persons, is designed generally to prevent a shift of tax burden to a trust from a beneficiary or beneficiaries. In the case of a foreign trust created by a U.S. person, subpart D is designed to prevent certain other tax avoidance possibilities. To accomplish these ends, subpart D provides special rules for treatment of amounts paid, credited, or required to be distributed by a complex trust (subject to subpart C (section 661 and following) of such part I) in any year in excess of distributable net income for that year. Such an excess distribution is defined as an accumulation distribution, subject to the limitations in section 665 (b) or (c). An accumulation distribution, in the case of a trust other than a foreign trust created by a U.S. person, is "thrown back" to each of the 5 preceding years in inverse order. In the case of a foreign trust created by a U.S. person such an accumulation distribution is "thrown back," in inverse order, to each of the preceding years to which the Internal Revenue Code of 1954 applies. That is, an accumulation distribution will be taxed to the beneficiaries of the trust in the year the distribution is made or required, but, in general, only to the extent of the distributable net income of those years which was not in fact distributed. However, with respect to a distribution by a trust other than a foreign trust created by a U.S. person, the resulting tax will not be greater than the aggregate of the taxes that would have been attributable to the amount thrown back to previous years had they been included in gross income of the beneficiaries in those years. In the case of a foreign trust created by a U.S. person, the resulting tax is computed under the provisions of section 669. To prevent double taxation, both in the case of a foreign trust created by a U.S. person, and a trust other than a foreign trust created by a U.S. person, the beneficiaries receive a credit for any taxes previously paid by the trust which are attributable to the excess thrown back and which are creditable under the provisions of chapter 1 of the Internal Revenue Code. Subpart D does not apply to any estate.
26 C.F.R. §1.665(a)-0