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 Montana (MT), common law refers to the body of law that is developed through court decisions rather than through statutes enacted by the legislature. When Montana's courts face legal issues that are not covered by state statutes or federal law, they rely on judicial precedents set by previous court rulings. These precedents are based on the principle of stare decisis, which obligates courts to follow the legal principles established by higher courts within the same jurisdiction, such as the Montana Supreme Court or the U.S. Supreme Court, to ensure consistency and predictability in the law. If a case presents an issue of first impression, meaning it has not been previously addressed by statute or relevant case law, Montana courts will decide the case by drawing analogies to similar legal principles and considering concepts of fairness and justice. This common law system in Montana is a legacy of the English legal tradition and plays a crucial role in shaping the state's legal landscape in areas not governed by statutory law.