Common law is a judge made law. It is the law that exists and applies on the basis of historical legal examples that were developed over hundreds of years. The common law is also known as "unwritten law" as it is written by judges and not by elected politicians. It is based on the traditional English law. All the states in the US (except Louisiana that follows French system) use a system of common law. It has been referred to as the "common sense of the community, crystallized and formulated by our ancestors".

The common law applies only to civil cases. In a criminal case the person cannot be convicted unless it is proved that a law that is written in the "law books" has been violated. In general the common law is understood as a contrast to the "statutory", and sometimes also to "equitable" or to "criminal" law.