26 U.S.C. § 2651: Generation assignment

For purposes of this chapter, the generation to which any person (other than the transferor) belongs shall be determined in accordance with the rules set forth in this section. An individual who is a lineal descendant of a grandparent of the transferor shall be assigned to that generation which results from comparing the number of generations between the grandparent and such individual with the number of generations between the grandparent and the transferor. An individual who is a lineal descendant of a grandparent of a spouse (or former spouse) of the transferor (other than such spouse) shall be assigned to that generation which results from comparing the number of generations between such grandparent and such individual with the number of generations between such grandparent and such spouse. A relationship by legal adoption shall be treated as a relationship by blood. A relationship by the half-blood shall be treated as a relationship of the whole-blood. An individual who has been married at any time to the transferor shall be assigned to the transferor’s generation. An individual who has been married at any time to an individual described in subsection (b) shall be assigned to the generation of the individual so described. an individual born not more than 12½ years after the date of the birth of the transferor assigned to the transferor’s generation, an individual born more than 12½ years but not more than 37½ years after the date of the birth of the transferor assigned to the first generation younger than the transferor, and similar rules for a new generation every 25 years. an individual is a descendant of a parent of the transferor (or the transferor’s spouse or former spouse), and such individual’s parent who is a lineal descendant of the parent of the transferor (or the transferor’s spouse or former spouse) is dead at the time the transfer (from which an interest of such individual is established or derived) is subject to a tax imposed by chapter 11 or 12 upon the transferor (and if there shall be more than 1 such time, then at the earliest such time), This subsection shall not apply with respect to a transfer to any individual who is not a lineal descendant of the transferor (or the transferor’s spouse or former spouse) if, at the time of the transfer, such transferor has any living lineal descendant. Except as provided in regulations, an individual who, but for this subsection, would be assigned to more than 1 generation shall be assigned to the youngest such generation. Except as provided in paragraph (3), if an estate, trust, partnership, corporation, or other entity has an interest in property, each individual having a beneficial interest in such entity shall be treated as having an interest in such property and shall be assigned to a generation under the foregoing provisions of this subsection. organization described in section 511(a)(2), charitable trust described in section 511(b)(2), and governmental entity,