Article IV, Section 1's Full Faith and Credit Clause requires states to recognize other states' "public Acts, Records, and judicial Proceedings," with Congress authorized to prescribe how these acts are proved and their legal effect. Public acts include state statutes, constitutions, and administrative regulations—the official legal outputs of state governments.
In 1790, Congress passed legislation requiring states to authenticate acts with official seals and clerk attestations.
The clause prevents states from ignoring each other's laws in most circumstances, though courts have held it's "less demanding" for choice-of-law questions than for final court judgments. Nevada can't void a California driver's license just because the holder crossed state lines, and states must honor professional licenses issued elsewhere under interstate compacts. However, states retain some discretion to apply their own law when competing state interests clash.