1. You are married and living together as spouses.
2. You are living together in a common law marriage that is recognized in the state where you now live or in the state where the common law marriage began.
3. You are married and living apart, but not legally separated under a decree of divorce or separate maintenance.
4. You are separated under an interlocutory (not final) decree of divorce. For purposes of filing a joint return, you are not considered divorced.

You are considered unmarried for the whole year if, on the last day of your tax year, you are unmarried or legally separated from your spouse under a divorce or separate maintenance decree. State law governs whether you are married or legally separated under a divorce or separate maintenance decree. 

See IRS Publication 17 Your Federal Income Tax (For Individuals) for more information.