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
With the transition from English law, which had common law crimes, to the new legal system under the U.S. Constitution, which prohibited "ex post facto" laws at both the federal and state level, the question was raised whether there could be common law crimes in the United States. It was settled in the case of "United States v. Hudson", which decided that federal courts had no jurisdiction to define new common law crimes, and that there must always be a (constitutional) statute defining the offense and the penalty for it.