The gold standard is a monetary system in which the standard economic unit of account is a fixed weight of gold. Three distinct kinds of gold standard can be identified. The gold specie standard is a system in which the monetary unit is associated with circulating gold coins, or with the unit of value defined in terms of one particular circulating gold coin in conjunction with subsidiary coinage made from a lesser valuable metal. The gold exchange standard may involve only the circulation of silver coins, or coins made of other metals, but the authorities will have guaranteed a fixed exchange rate with another country that is on the gold standard, hence creating a de facto gold standard, in that the value of the silver coins has a fixed external value in terms of gold that is independent of the inherent silver value. The gold bullion standard is a system in which gold coins do not actually circulate as such, but in which the authorities have agreed to sell gold bullion on demand at a fixed price in exchange for the circulating currency.