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.
Celtic law
It is problematic to date ‘Common Celtic law’, and the best we can arrive at are rough estimates. It is quite apparent, by parallels existing between Celtic and other Indo-European laws that at least some of the legal principles that make up Common Celtic law must be very ancient, perhaps going back to the Early Bronze Age or even the Neolithic period. Others may have only developed much later, perhaps even only as a result of contacts with Mediterranean cultures (mainly Greeks and Romans) during the late Iron Age and Antiquity.