Explanation:

 A gold standard is a monetary system in which the standard economic unit of account is based on a fixed quantity of gold. Three types can be distinguished: specie, bullion, and exchange.
-In the gold specie standard the monetary unit is associated with the value of circulating gold coins or the monetary unit has the value of a certain circulating gold coin, but other coins may be made of less valuable metal.
-The gold bullion standard is a system in which gold coins do not circulate, but the authorities agree to sell gold bullion on demand at a fixed price in exchange for the circulating currency.
-The gold exchange standard usually does not involve the circulation of gold coins. The main feature of the gold exchange standard is that the government guarantees a fixed exchange rate to the currency of another country that uses a gold standard (specie or bullion), regardless of what type of notes or coins are used as a means of exchange.
-A formal gold specie standard was first established in 1821, when Britain adopted it following the introduction of the gold sovereign by the new Royal Mint at Tower Hill in 1816.