26 CFR § 1.21-3 - Special rules applicable to married taxpayers.
(a) Joint return requirement. No credit is allowed under section 21 for taxpayers who are married (within the meaning of section 7703 and the regulations thereunder) at the close of the taxable year unless the taxpayer and spouse file a joint return for the taxable year. See section 6013 and the regulations thereunder relating to joint returns of income tax by husband and wife.
(b) Taxpayers treated as not married. The requirements of paragraph (a) of this section do not apply to a taxpayer who is legally separated under a decree of divorce or separate maintenance or who is treated as not married under section 7703(b) and the regulations thereunder (relating to certain married taxpayers living apart). A taxpayer who is treated as not married under this paragraph (b) is not required to take into account the earned income of the taxpayer's spouse for purposes of applying the earned income limitation on the amount of employment-related expenses under § 1.21-2(b).
(c) Death of married taxpayer. If a married taxpayer dies during the taxable year and the survivor may make a joint return with respect to the deceased spouse under section 6013(a)(3), the credit is allowed for the year only if a joint return is made. If, however, the surviving spouse remarries before the end of the taxable year in which the deceased spouse dies, a credit may be allowed on the decedent spouse(s separate return.