26 U.S.C. § 2653: Taxation of multiple skips

there is a generation-skipping transfer of any property, and immediately after such transfer such property is held in trust, Except as provided in paragraph (2), the provisions of subsection (a) shall not affect the inclusion ratio determined with respect to any trust. Under regulations prescribed by the Secretary, notwithstanding the preceding sentence, proper adjustment shall be made to the inclusion ratio with respect to such trust to take into account any tax under this chapter borne by such trust which is imposed by this chapter on the transfer described in subsection (a). If the generation-skipping transfer referred to in subsection (a) involves the transfer of property from 1 trust to another trust (hereinafter in this paragraph referred to as the “pour-over trust”), the inclusion ratio for the pour-over trust shall be determined by treating the nontax portion of such distribution as if it were a part of a GST exemption allocated to such trust. For purposes of subparagraph (A), the nontax portion of any distribution is the amount of such distribution multiplied by the applicable fraction which applies to such distribution.