The Full Faith and Credit Clause is the section of the U.S. Constitution that ensures the judicial system of one state cooperates with the judicial systems of other states. This prevents judicial systems from being required to repeat judicial proceedings.
The Full Faith and Credit Clause is necessary for the cohesion of the union of different states. A similar clause was present in the Articles of Confederation, which played a role similar to the modern U.S. Constitution before the Constitution was ratified. The Supreme Court of the United States has cited the clause in a variety of cases involving state-court systems.