Most of the legal rules that affect us are passed by the legislature, and signed by the governor. However, judges can make rules too by deciding cases. This is called common law.

In this system, a judge makes their decision based on past decisions from similar cases. These past decisions are known as precedent decisions. Judges follow precedent decisions. This ensures that the law is constant.

Judges may overrule a past decision, but this is rare. When they overrule a past decision, the judge explains why. A judge's reasons may be because society has changed, or because the facts in this case are different from past cases.

When a precedent decision is overruled, the judge can make a new decision on the case that either makes an existing rule bigger or smaller. The new decision can also make a new rule. The new decision is called a landmark decision or leading case in later cases.

Common law between courts

In the state and federal court systems, there are three levels of courts: trial, appellate, and supreme. In Illinois, the trial courts are called “circuit courts.” In the federal system, trial courts are called “district courts.”

Trial court decisions can be appealed to appellate courts. Appellate courts are divided up to only cover certain areas. Decisions of appellate courts can be appealed to state supreme courts, or the US Supreme Court.

The decision of a higher court becomes a binding precedent for lower courts. This means that any precedent decisions made by the US Supreme Court must be followed by all courts. Precedent decisions by one US appellate court must be followed by the district courts in that appellate court’s area.

The decision of a US appellate court is not binding to other US appellate courts. It only has persuasive influence. This means that other courts will consider it, but can still disagree with it.

Examples of common law in use today

You might have heard of common law marriage. Although this doesn't exist anymore in Illinois, it does in other states. This is an example of how common law still affects us today.

Another example is the law of tort. This covers any lawsuit where someone is suing someone else because of an injury or damage that happened to their property. When someone hits someone else with their car, the person who was hit can sue the other person for a tort, usually negligence. Many of the rules about what counts as negligence have been made over time through the process of common law.