Guinea was the principal English gold coin during the period of legal bimetalism, from 1663 until the Act of 1816. Originally issued to pass as the legal equivalent of 20 shillings of silver, this ratio for many years overvalued silver so that the guinea passed at a premium. In 1717, a Royal decree forbade anyone to receive a guinea at any rate or value higher than 21 shillings. Since the Act of 1816, the guinea has become merely a nominal term for 21 shillings. Certain "quality" goods and services are still quoted in guineas rather than pounds sterling.
- Percy L. Greaves, Jr. "Mises Made Easier ", 1974. Referenced 2014-07-15.