To understand the function and scope of state courts, it is necessary to consider them in relation to the federal court system expressly created in Article III of the U.S. Constitution. Article III also establishes the type of cases that federal courts may hear and decide (federal “jurisdiction”).
Article VII of the Constitution declares that “This Constitution, and the Laws of the United States … and all Treaties made … under the Authority of the United States, shall be the supreme Law of the Land; and the Judges in every State shall be bound thereby, any Thing in the Constitution or Laws of any State to the Contrary notwithstanding.” Later in the Constitution, the Tenth Amendment provides that “powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.”
The ultimate effect these provisions have upon state courts is to reserve to them the right to hear and decide any legal matter not expressly reserved for the exclusive jurisdiction of federal courts (such as lawsuits between states). This matter mostly involves the “adjudication” of controversies concerning state laws, which impact the daily lives of citizens in a much greater manner than federal laws. State courts may also rule upon certain issues concerning federal law and the federal Constitution.
State legislatures are therefore free to create—and state courts are free to enforce—any law, regulation, or rule that does not conflict with or abridge the guarantees of the federal Constitution (or the state’s own constitution). The wide variance, from state to state, of both structure and procedure within the court systems is precisely due to the preservation of those independent powers to the states by the U.S. Constitution.