Common law
The common lawso named because it was "common" to all the king's courts across Englandoriginated in the practices of the courts of the English kings in the centuries following the Norman Conquest in 1066. The British Empire spread the English legal system to its historical colonies, many of which retain the common law system today. These "common law systems" are legal systems that give great precedential weight to common law, and to the style of reasoning inherited from the English legal system.
Common law copyright
"Common law copyright" is also the term used in the United States to refer to most state law copyright claims. In 1978, Section 301 took effect, preempting all state common law copyright claims that fall under subject matter in Section 102 (Subject matter of copyright: In general) or Section 103 (Subject matter of copyright: Compilations and derivative works) except for sound recordings fixed before February 15, 1972. This leaves a sizable amount of work that still falls under a mixture of state statutes and common law copyright.