(a) In General. This section provides rules, in addition to those prescribed in § 1.904-2, for the carryback and carryover of the unused foreign tax paid or accrued to a foreign country or possession by spouses making a joint return for one or more of the taxable years involved in the computation of the carryback or carryover. The rules in this section apply separately with respect to each separate category as defined in § 1.904-5(a)(4)(v).

(b) Joint unused foreign tax and joint excess limitation. In the case of spouses the joint unused foreign tax or the joint excess limitation for a taxable year for which a joint return is made shall be computed on the basis of the combined income, deductions, taxes, and credit of both spouses as if the combined income, deductions, taxes, and credit were those of one individual.

(c) Continuous use of joint return. If spouses make a joint return for the current taxable year, and also make joint returns for each of the other taxable years involved in the computation of the carryback or carryover of the unused foreign tax to the current taxable year, the joint carryback or the joint carryover to the current taxable year shall be computed on the basis of the joint unused foreign tax and the joint excess limitations.

(d) From separate to joint return. If spouses make a joint return for the current taxable year, but make separate returns for all of the other taxable years involved in the computation of the carryback or carryover of the unused foreign tax to the current taxable year, the separate carrybacks or separate carryovers shall be a joint carryback or a joint carryover to the current taxable year.

(e) Amounts carried from or through a joint return year to or through a separate return year—(1) In general. It is necessary to allocate to each spouse the spouse's share of an unused foreign tax or excess limitation for any taxable year for which the spouses filed a joint return if—

(i) The spouses file separate returns for the current taxable year and an unused foreign tax is carried thereto from a taxable year for which they filed a joint return;

(ii) The spouses file separate returns for the current taxable year and an unused foreign tax is carried to such taxable year from a year for which they filed separate returns but is first carried through a year for which they filed a joint return; or

(iii) The spouses file a joint return for the current taxable year and an unused foreign tax is carried from a taxable year for which they filed joint returns but is first carried through a year for which they filed separate returns.

(2) Computation and adjustments. In the cases described in paragraph (e)(1) of this section, the separate carryback or carryover of each spouse to the current taxable year shall be computed in the manner described in § 1.904-2 but with the modifications set forth in paragraph (f) of this section. Where applicable, appropriate adjustments are made to take into account the fact that, for any taxable year involved in the computation of the carryback or the carryover, either spouse has combined foreign oil and gas income described in section 907(b) with respect to which the limitation in section 907(a) applies.

(f) Allocation of unused foreign tax and excess limitation— (1) Separate category limitation. The limitation in a separate category of a particular spouse for a taxable year for which a joint return is made shall be the portion of the limitation on the joint return which bears the same ratio to such limitation as such spouse's foreign source taxable income (with gross income and deductions taken into account to the same extent as taken into account on the joint return) in such separate category (but not in excess of the joint foreign source taxable income) bears to the joint foreign source taxable income in such separate category.

(2) Unused foreign tax. For purposes of this section, the term unused foreign tax means, with respect to a particular spouse and separate category for a taxable year for which a joint return is made, the excess of the foreign tax paid or accrued by that spouse with respect to that separate category over that spouse's separate category limitation.

(3) Excess limitation. For purposes of this section, the term excess limitation means, with respect to a particular spouse and separate category for a taxable year for which a joint return is made, the excess of that spouse's separate category limitation over the foreign taxes paid or accrued by such spouse with respect to such separate category for such taxable year.

(4) Excess limitation to be applied. The excess limitation of the particular spouse for any taxable year which is applied against the unused foreign tax of that spouse for another taxable year in order to determine the amount of the unused foreign tax which shall be carried back or over to a third taxable year shall be, in a case in which the excess limitation is determined on a joint return, the sum of the following amounts:

(i) Such spouse's excess limitation determined under subparagraph (3) of this paragraph reduced as provided in subparagraph (5)(i) of this paragraph, and

(ii) The excess limitation of the other spouse determined under subparagraph (3) of this paragraph for that taxable year reduced as provided in subparagraphs (5) (i) and (ii) of this paragraph.

(5) Reduction of excess limitation. (i) The part of the excess limitation which is attributable to each spouse for the taxable year, as determined under subparagraph (3) of this paragraph, shall be reduced by absorbing as taxes deemed paid or accrued under section 904(c) in that year the unabsorbed separate unused foreign tax of such spouse, and the unabsorbed unused foreign tax determined under subparagraph (2) of this paragraph of such spouse, for taxable years which begin before the beginning of the year of origin of the unused foreign tax of the particular spouse against which the excess limitation so determined is being applied.

(ii) In addition, the part of the excess limitation which is attributable to the other spouse for the taxable year, as determined under subparagraph (3) of this paragraph, shall be reduced by absorbing as taxes deemed paid or accrued under section 904(c) in that year the unabsorbed unused foreign tax, if any, of such other spouse for the taxable year which begins on the same date as the beginning of the year of origin of the unused foreign tax of the particular spouse against which the excess limitation so determined is being applied.

(g) [Reserved]

(h) Applicability date. This section is applicable for taxable years that both begin after December 31, 2017, and end on or after December 4, 2018.