26 U.S.C. § 7703 — Determination of marital status

(a) General rule For the tax rules in part V of subchapter B of chapter 1, and any other provision that points to this subsection: (1) Whether you're married is decided as of the last day of your tax year — except that if your spouse dies during the year, the determination is made as of the moment of death instead. (2) If you're legally separated from your spouse under a decree of divorce or of separate maintenance, you are not considered married. (b) Certain married individuals living apart For any tax provision that points to this subsection, you are treated as not married — even though you're still married under subsection (a) — if all three of these are true for the taxable year: (1) you're married, you file a separate return, and for more than half the year your home is the main home of a child (within the meaning of section 152(f)(1)) for whom you're entitled to a deduction under section 151 (or would be, except for section 152(e)); (2) you pay more than half the cost of keeping up that household during the year; and (3) your spouse is not a member of that household at any point during the last 6 months of the year.