A brass (copper and zinc) Roman coin (sesterius singular, sestertii plural). These are the largest of the Roman coins at about 28 to 32mm in diameter. They were used as a basic unit of exchange and are therefore quite common finds. 3rd century AD sestertii often appear more square in shape than those produced in the 1st or 2nd century AD. 100 sesterii were worth the same as twenty five denarii.