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The United States legal system developed primarily out of the English common law system (with the exception of the state of Louisiana, which continued to follow the French civilian system after being admitted to statehood). Some concepts which originate in Spanish law, such as the prior appropriation doctrine and community property, still persist in some U.S. states, particularly those which were part of the Mexican Cession in 1848.

Under the doctrine of federalism, each state has its own separate court system, and the ability to legislate within areas not reserved to the federal government.