(a) Under section 673(a), a grantor, in general, is treated as the owner of any portion of a trust in which he has a reversionary interest in either the corpus or income if, as of the inception of that portion of the trust, the grantor's interest will or may reasonably be expected to take effect in possession or enjoyment within 10 years commencing with the date of transfer of that portion of the trust. However, the following types of reversionary interests are excepted from the general rule of the preceding sentence:

(1) A reversionary interest after the death of the income beneficiary of a trust (see paragraph (b) of this section); and

(2) Except in the case of transfers in trust made after April 22, 1969, a reversionary interest in a charitable trust meeting the requirements of section 673(b) (see § 1.673(b)-1). Even though the duration of the trust may be such that the grantor is not treated as its owner under section 673, and therefore is not taxed on the ordinary income, he may nevertheless be treated as an owner under section 677(a)(2) if he has a reversionary interest in the corpus. In the latter case, items of income, deduction, and credit allocable to corpus, such as capital gains and losses, will be included in the portion he owns. See § 1.671-3 and the regulations under section 677. See § 1.673(d)-1 with respect to a postponement of the date specified for reacquisition of a reversionary interest.

(b) Section 673(c) provides that a grantor is not treated as the owner of any portion of a trust by reason of section 673 if his reversionary interest in the portion is not to take effect in possession or enjoyment until the death of the person or persons to whom the income of the portion is regardless of the life expectancies of the income beneficiaries. If his reversionary interest is to take effect on or after the death of an income beneficiary or upon the expiration of a specific term of years, whichever is earlier, the grantor is treated as the owner if the specific term of years is less than 10 years (but not if the term is 10 years or longer).

(c) Where the grantor's reversionary interest in a portion of a trust is to take effect in possession or enjoyment by reason of some event other than the expiration of a specific term of years or the death of the income beneficiary, the grantor is treated as the owner of the portion if the event may reasonably be expected to occur within 10 years from the date of transfer of that portion, but he is not treated as the owner under section 673 if the event may not reasonably be expected to occur within 10 years from that date. For example, if the reversionary interest in any portion of a trust is to take effect on or after the death of the grantor (or any person other than the person to whom the income is payable) the grantor is treated under section 673 as the owner of the portion if the life expectancy of the grantor (or other person) is less than 10 years on the date of transfer of the portion, but not if the life expectancy is 10 years or longer. If the reversionary interest in any portion is to take effect on or after the death of the grantor (or any person other than the person to whom the income is payable) or upon the expiration of a specific term of years, whichever is earlier, the grantor is treated as the owner of the portion if on the date of transfer of the portion either the life expectancy of the grantor (or other person) or the specific term is less than 10 years; however, if both the life expectancy and the specific term are 10 years or longer the grantor is not treated as the owner of the portion under section 673. Similarly, if the grantor has a reversionary interest in any portion which will take effect at the death of the income beneficiary or the grantor, whichever is earlier, the grantor is not treated as an owner of the portion unless his life expectancy is less than 10 years.

(d) It is immaterial that a reversionary interest in corpus or income is subject to a contingency if the reversionary interest may, taking the contingency into consideration, reasonably be expected to take effect in possession or enjoyment within 10 years. For example, the grantor is taxable where the trust income is to be paid to the grantor's son for 3 years, and the corpus is then to be returned to the grantor if he survives that period, or to be paid to the grantor's son if he is already decreased.

(e) See section 671 and §§ 1.671-2 and 1.671-3 for rules for treatment of items of income, deduction, and credit when a person is treated as the owner of all or only a portion of a trust.