Common law consists of decisions by courts (judicial decisions) that do not involve interpretation of statutes, regulations, treaties, or the Constitution. Courts make such interpretations, but many cases are decided where there is no statutory or other codified law or regulation to be interpreted. For example, a state court deciding what kinds of witnesses are required for a valid will in the absence of a rule (from a statute) is making common law.

United States law comes primarily from the tradition of English common law. By the time England’s American colonies revolted in 1776, English common-law traditions were well established in the colonial courts. English common law was a system that gave written judicial decisions the force of law throughout the country. Thus if an English court delivered an opinion as to what constituted the common-law crime of burglary, other courts would stick to that decision, so that a common body of law developed throughout the country. Common law is essentially shorthand for the notion that a common body of law, based on past written decisions, is desirable and necessary.

In England and in the laws of the original thirteen states, common-law decisions defined crimes such as arson, burglary, homicide, and robbery. As time went on, US state legislatures either adopted or modified common-law definitions of most crimes by putting them in the form of codes or statutes. This legislative ability—to modify or change common law into judicial law—points to an important phenomenon: the priority of statutory law over common law. As we will see in the next section, constitutional law will have priority over statutory law.