The sestertius, or sesterce, (pl. sestertii or sesterces) was an ancient Roman coin. During the Roman Republic it was a small, silver coin issued only on rare occasions. During the Roman Empire it was a large brass coin. The name sestertius means "two and one half", referring to its nominal value of two and a half asses, a value that was useful for commerce because it was one quarter of a denarius, a coin worth ten asses. The name is derived from semis, "half" and "tertius", "third", in which "third" refers to the third as: the sestertius was worth two full asses and half of a third.