(a) The applicability of the separate share rule provided by section 663(c) to trusts other than qualified revocable trusts within the meaning of section 645(b)(1) will generally depend upon whether distributions of the trust are to be made in substantially the same manner as if separate trusts had been created. Thus, if an instrument directs a trustee to divide the testator's residuary estate into separate shares (which under applicable law do not constitute separate trusts) for each of the testator's children and the trustee is given discretion, with respect to each share, to distribute or accumulate income or to distribute principal or accumulated income, or to do both, separate shares will exist under section 663(c). In determining whether separate shares exist, it is immaterial whether the principal and any accumulated income of each share is ultimately distributable to the beneficiary of such share, to his descendants, to his appointees under a general or special power of appointment, or to any other beneficiaries (including a charitable organization) designated to receive his share of the trust and accumulated income upon termination of the beneficiary's interest in the share. Thus, a separate share may exist if the instrument provides that upon the death of the beneficiary of the share, the share will be added to the shares of the other beneficiaries of the trust.

(b) Separate share treatment will not be applied to a trust or portion of a trust subject to a power to: (1) Distribute, apportion, or accumulate income, or (2) distribute corpus to or for one or more beneficiaries within a group or class of beneficiaries, unless payment of income, accumulated income, or corpus of a share of one beneficiary cannot affect the proportionate share of income, accumulated income, or corpus of any shares of the other beneficiaries, or unless substantially proper adjustment must thereafter be made (under the governing instrument) so that substantially separate and independent shares exist.

(c) A share may be considered as separate even though more than one beneficiary has an interest in it. For example, two beneficiaries may have equal, disproportionate, or indeterminate interests in one share which is separate and independent from another share in which one or more beneficiaries have an interest. Likewise, the same person may be a beneficiary of more than one separate share.

(d) Separate share treatment may be given to a trust or portion of a trust otherwise qualifying under this section if the trust or portion of a trust is subject to a power to pay out to a beneficiary of a share (of such trust or portion) an amount of corpus in excess of his proportionate share of the corpus of the trust if the possibility of exercise of the power is remote. For example, if the trust is subject to a power to invade the entire corpus for the health, education, support, or maintenance of A, separate share treatment is applied if exercise of the power requires consideration of A's other income which is so substantial as to make the possibility of exercise of the power remote. If instead it appears that A and B have separate shares in a trust, subject to a power to invade the entire corpus for the comfort, pleasure, desire, or happiness of A, separate share treatment shall not be applied.

(e) For taxable years ending before December 31, 1978, the separate share rule may also be applicable to successive interests in point of time, as for instance in the case of a trust providing for a life estate to A and a second life estate or outright remainder to B. In such a case, in the taxable year of a trust in which a beneficiary dies items of income and deduction properly allocable under trust accounting principles to the period before a beneficiary's death are attributed to one share, and those allocable to the period after the beneficiary's death are attributed to the other share. Separate share treatment is not available to a succeeding interest, however, with respect to distributions which would otherwise be deemed distributed in a taxable year of the earlier interest under the throwback provisions of subpart D (section 665 and following), part I, subchapter J, chapter 1 of the Code. The application of this paragraph may be illustrated by the following example: