The Full Faith and Credit clause states that the courts must honor out-of-state laws, regulations, and judgments. Thus, if a couple is married under the laws of one state, the marriage must be given full faith and credit in all other states. It was intended to prevent states from overruling the laws of other states when they come in contact with the laws of that state.
Article IV, Section 1:
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.
United States Library of Congress, The Constitution of the United States of America: Analysis and Interpretation
The historical background of the Full Faith and Credit Clause is furnished by the branch of private law that is variously termed "private international law," "conflict of laws", and "comity." This branch comprises a body of rules, based largely on the writings of jurists and judicial decisions, in accordance with which the courts of one country, or "jurisdiction," will ordinarily, in the absence of a local policy to the contrary, extend recognition and enforcement to rights claimed by individuals by virtue of the laws or judicial decisions of another country or "jurisdiction."
The most frequently applied examples of these rules include the following:
Still other rules, of first importance in the present connection, determine the recognition that the judgments of the courts of one country shall receive from those of another country.
So, even had the states of the Union remained in a mutual relationship of entire independence, private claims originating in one often would have been assured recognition and enforcement in the others. The Framers felt, however, that the rules of private international law should not be left among the states altogether on a basis of comity and hence subject always to the overruling local policy of the lex fori, but ought to be in some measure at least placed on the higher plane of constitutional obligation. In fulfillment of this intent, the Full Faith and Credit Clause was inserted, and Congress was empowered to enact supplementary and enforcing legislation.2
Under the present system, suit ordinarily must be brought where the defendant, the alleged wrongdoer, resides, which means generally where no part of the transaction giving rise to the action took place. What could be more irrational? "Granted that no state can of its own volition make its process run beyond its borders . . . is it unreasonable that the United States should by federal action be made a unit in the manner suggested?"3
Indeed, there are few clauses of the Constitution, the merely literal possibilities of which have been so little developed as the Full Faith and Credit Clause. Congress has the power under the clause to decree the effect that the statutes of one state shall have in other states. This being so, it does not seem extravagant to argue that Congress may under the clause describe a certain type of divorce and say that it shall be granted recognition throughout the Union and that no other kind shall. Or to speak in more general terms, Congress has under the clause power to enact standards whereby uniformity of state legislation may be secured as to almost any matter in connection with which interstate recognition of private rights would be useful and valuable.
Doubtless Congress, by virtue of its powers in the field of foreign relations, might also lay down a mandatory rule regarding recognition of foreign judgments in every court of the United States. At present, the duty to recognize judgments even in national courts rests only on comity and is qualified in the judgment of the Supreme Court, by a strict rule of parity.4