The silver standard is a monetary system in which the standard economic unit of account is a fixed weight of silver. Silver was far more widespread than gold as the monetary standard worldwide, from the Sumerians c. 3000 BC until 1873. Following the discovery in the 16th century of large deposits of silver at the Cerro Rico in Potosí, Bolivia, an international silver standard came into existence in conjunction with the Spanish pieces of eight. These silver dollar coins were an international trading currency for nearly four hundred years.