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.
Law of the United States
At both the federal and state levels, with the exception of the state of Louisiana, the law of the United States is largely derived from the common law system of English law, which was in force at the time of the American Revolutionary War. However, American law has diverged greatly from its English ancestor both in terms of substance and procedure, and has incorporated a number of civil law innovations.