Common law—also known as case law, court opinions, judicial precedent, or judge-made law—is the body of law within each state or federal circuit that is created when judges write decisions (court opinions) resolving lawsuits—and in doing so sometimes apply legal principles that are not included in the state or federal statutes. When the state legislature or the U.S. Congress has not passed a law (state or federal statute) governing a certain legal issue, the courts look to traditional (and sometimes new) legal principles to resolve those issues. And those principles—as stated in case law—are the common law.

The concept of common law is derived from the English system and is based on precedent—the manner in which courts have previously resolved a similar legal dispute involving similar factual circumstances. If a similar legal dispute involving similar facts has been resolved in a certain way in the past, a current court is generally required to follow the prior application of the law under the doctrine of stare decisis—a Latin term pronounced “starry duh-sigh-suss”—to maintain consistency and predictability in the law. But a court’s obligation to follow precedent is generally limited to case law from higher courts (generally the state supreme court or the U.S. Supreme Court) or from the same court (district court of appeals or federal circuit court, for example).

And if a court determines a dispute involves a legal issue that is not addressed in a statute or in a previous court opinion from the same state or jurisdiction (and thus is a matter of first impression) the court will resolve the dispute as best it can—often by analogy to similar statutes and legal principles—as well as the more subjective ideas of fairness and justice.

In Illinois, common law, also known as case law or judicial precedent, plays a significant role in the legal system. It consists of legal principles and rulings made by judges when they resolve lawsuits. When Illinois state statutes or federal laws do not cover a specific legal issue, Illinois courts rely on common law to fill in the gaps. This body of law is based on the doctrine of stare decisis, which requires courts to follow legal precedents set by higher courts, such as the Illinois Supreme Court or the U.S. Supreme Court, to ensure consistency and predictability in the law. However, when an Illinois court faces a legal question that has not been previously addressed (a matter of first impression), the court will make a determination using analogous statutes, legal principles, and considerations of fairness and justice to reach a decision.