Relationships between state and federal courts

Separate from, but not entirely independent of, this federal court system are the court systems of each state, each dealing with, in addition to federal law when not deemed preempted, a state's own laws, and having its own court rules and procedures. Although state governments and the federal government are legally dual sovereigns, the Supreme Court of the United States is in many cases the appellate court from the State Supreme Courts (e.g., absent the Court countenancing the applicability of the doctrine of adequate and independent State grounds). The Supreme Courts of each state are by this doctrine the final authority on the interpretation of the applicable state's laws and Constitution. Many state constitution provisions are equal in breadth to those of the U.S. Constitution, but are considered "parallel" (thus, where, for example, the right to privacy pursuant to a state constitution is broader than the federal right to privacy, and the asserted ground is explicitly held to be "independent", the question can be finally decided in a State Supreme Court—the U.S. Supreme Court will decline to take jurisdiction).

A State Supreme Court, other than of its own accord, is bound only by the U.S. Supreme Court's interpretation of federal law, but is not bound by interpretation of federal law by the federal court of appeals for the federal circuit in which the state is included, or even the federal district courts located in the state, a result of the dual sovereigns concept. Conversely, a federal district court hearing a matter involving only a question of state law (usually through diversity jurisdiction) must apply the substantive law of the state in which the court sits, a result of the application of the Erie Doctrine; however, at the same time, the case is heard under the Federal Rules of Civil Procedure, the Federal Rules of Criminal Procedure and the Federal Rules of Evidence instead of state procedural rules (that is, the application of the Erie Doctrine only extends to a requirement that a federal court asserting diversity jurisdiction apply substantive state law, but not procedural state law, which may be different). Together, the laws of the federal and state governments form U.S. law.