The British gold coin worth 21 shillings was the guinea.
The Royal Mint first struck the guinea in 1663, using gold associated with the Guinea Coast of West Africa. Its original value was 20 shillings, but the coin’s market value rose as the price of gold changed. By 1717, the guinea was officially valued at 21 shillings.
Guineas became especially associated with professional fees, horse sales, and luxury goods. A price of “five guineas,” for example, meant five guineas, or £5 5s. This usage survived even after the coin itself disappeared.
The guinea was replaced in ordinary circulation by the sovereign and half-sovereign. The Royal Mint stopped producing guineas in 1814, although the guinea continued as a unit of account in Britain for many decades. It should not be confused with the later sovereign, which was worth one pound.