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There are three bases of American law:

  1. Case law: Court decisions that inform judicial rulings
  2. Constitutions: Agreements, such the U.S. Constitution and the state constitutions, that outline the structure of government
  3. Statutes: Laws made by governments

Case Law

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.

Overturning Precedent

Judges rely on precedents when deciding their cases, but the Supreme Court also has the power to overturn precedents. Some of the most famous cases in American legal history have overturned precedents. For example, the civil rights case Brown v. Board of Education (1954), which outlawed segregation in public schools, overturned the 1896 case Plessy v. Ferguson. State and federal courts are reluctant to overturn precedent because the law needs to be stable for the courts to have legitimacy. It would be impossible for anyone to obey a law that kept changing. At the same time, judges recognize that the law must change to stay relevant. The courts need to strike a balance between stability and change.

Constitutions

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.

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