US Constitution Series, Article IV, Section 1
Article IV. STATES' RELATIONSThis section will be the battleground in the gay marriage controversy.
§ 1. Full Faith and Credit
Full Faith and Credit shall be given in each State to the public Acts, Records, and judicial Proceedings of every other State. And the Congress may by general Laws prescribe the Manner in which such Acts, Records and Proceedings shall be proved, and the Effect thereof.