an old French coin, originally made of gold and later of silver and copper. One sou was equal to 1/20 of a livre or to 12 deniers. When France adopted the decimal system in 1799, the sou was replaced by a 5-centime coin equal to 1/20 of a livre or of a franc. The new coin was commonly called a sou until 1947, when it was withdrawn from circulation in connection with the devaluation of the French franc.