26 CFR § 20.2013-3 - “Second limitation”.
(a) The amount of the Federal estate tax attributable to the transferred property in the present decedent's estate is the “second limitation”. Thus, the credit is limited to the difference between—
(1) The net estate tax payable (see paragraph (b)(5) or (c), as the case may be, of § 20.0–2) with respect to the present decedent's estate, determined without regard to any credit for tax on prior transfers under section 2013 or any credit for foreign death taxes claimed under the provisions of a death tax convention, and
(2) The net estate tax determined as provided in subparagraph (1) of this paragraph but computed by subtracting from the present decedent's gross estate the value of the property transferred (see § 20.2013–4), and by making only the adjustment indicated in paragraph (b) of this section if a charitable deduction is allowable to the estate of the present decedent.
(b) If a charitable deduction is allowable to the estate of the present decedent under the provisions of section 2055 or section 2106 (a)(2) (for estates of nonresidents not citizens), for purposes of determining the tax described in paragraph (a)(2) of this section, the charitable deduction otherwise allowable is reduced by an amount, E, which bears the same ratio to F (the charitable deduction otherwise allowable) as G (the value of the transferred property (see § 20.2013–4)) bears to H (the value of the present decedent's gross estate reduced by the amount of the deductions for expenses, indebtedness, taxes, losses, etc., allowed under the provisions of sections 2053 and 2054 or section 2106(a)(1) (for estates of nonresidents not citizens)). See paragraph (c)(2) of example (1) and paragraph (c)(2) of example (2) in § 20.2013–6.
(c) If the credit for tax on prior transfers relates to property received from two or more transferors, the property received from all transferors is aggregated in determining the limitation on credit under this section (the “second limitation”). However, the limitation so determined is apportioned to the property received from each transferor in the ratio that the property received from each transferor bears to the total property received from all transferors. See paragraph (c) of example (2) in § 20.2013–6.
(d) For illustrations of the application of this section, see examples (1) and (2) set forth in § 20.2013–6.