Both the British Imperial and United States customary systems of measurement derive from earlier English systems used in the Middle Ages, that were the result of a combination of the local Anglo-Saxon units inherited from Germanic tribes and Roman units brought by William the Conqueror after the Norman Conquest of England in 1066. Having this shared heritage, the two systems are quite similar, but there are differences. The US customary system is based on English systems of the 18th century, while the Imperial system was defined in 1824, almost a half-century after American independence.
