There are three bases of American law:
- Case law: Court decisions that inform judicial rulings
- Constitutions: Agreements, such the U.S. Constitution and the state constitutions, that outline the structure of government
- Statutes: Laws made by governments
The American legal system has its roots in the British system, which is based on common law. In this system, judges shape the law through their decisions, interpretations, and rulings, which are then collected into a body of law known as case law that other judges can use as reference. When judges make decisions, they look to similar cases for precedent, a court ruling from the past similar to the current case. The Latin phrase stare decisis denotes the legal doctrine of relying on precedent.
Example: The Fourth Amendment states that citizens are protected from “unreasonable searches and seizures” and that search warrants can only be issued based on “probable cause.” Many cases have laid down rules about how the courts should handle such matters. In the case Mapp v. Ohio (1961), the Supreme Court applied the exclusionary rule—which states that any evidence obtained through an illegal search is excluded from trial—to state courts. Since then, judges have referred to the precedent set in Mapp v. Ohio to keep illegally obtained evidence out of the courtroom.
The U.S. Constitution is the supreme law of the land. No law or act of government—at the local, state, or federal level—can violate its principles. Similarly, a state’s constitution is the supreme law within the state’s borders, so long as the state constitution does not conflict with the national Constitution.