Case law in the United States

Case law (precedental law) is the body of law which regulates, via precedents, how laws are to be understood, based on how prior cases have been decided. Case law governs the impact court decisions have on future cases. This body of law requires courts be bound by their earlier decisions, and by the decisions of higher courts. According to the common law prinicple of stare decesis; a lower court should interpret laws in a manner consistent with a decision of higher courts. Generally, case law is understood as being established by the decisions of a Supreme Court or by various appellate courts.

Factually, case law can rest in the decisions of lower courts as well, especially within the jurisdictions of those lower courts, and when the issue in question has not been decided by a higher court. Among precedents, there are mandatory precedents and non-mandatory precedents. A mandatory precedent is one that a court should follow, while a non-mandatory precedent is one which the court may or may not follow. Though if a judge does not follow a mandatory precedent the only thing that will happen is that her decision will get overturned on appeal. If it is not appealed by one of the parties for a variety of reasons (cost, timeliness, bankruptcy) then the decision, even if it is against precedent, will stand. Sometimes judges will rule against precedent if they see a reason to overturn prior case law, perhaps to help the law evolve in changes social circumstances or to recognized more sophisticated legal reasoning.

In the United States, the United States Supreme Court hears cases, decides issues of legal intepretation and establishes precedent through published opinions only on cases related to federal law or the United States Constitution.

In the individual state judicial systems of the United States, the highest court may be styled State Supreme Court, Court of Appeals or other titles. These courts are empowered to render ultimate decisions on issues of state law or their respective state constitution. Although they may also rule on issues of federal law, their rulings should follow the precedents of federal courts in their state and may be appealled to the federal courts when there is an issue of federal law involved.

Under the United States Constitution a case may be brought to the federal courts only when there is either a federal issue, called federal question jurisdiction or when the issue involves persons from different states or countries, called diversity jurisdiction. If the federal court decision relies on the interpretation of a state law, the federal court should defer to the interpretation of the state courts when there is no federal law involved. Thus a decision by the United States Supreme Court on a matter of state law is not a mandatory precedent.