The Guinea is a coin of approximately one quarter ounce of gold that was minted in the Kingdom of England and later in the Kingdom of Great Britain and the United Kingdom between 1663 and 1814.
It was the first English machine-struck gold coin, originally worth one pound sterling, equal to twenty shillings; but rises in the price of gold relative to silver caused the value of the guinea to increase, at times to as high as thirty shillings; from 1717 until 1816, its value was officially fixed at twenty-one shillings. Following that, Great Britain adopted the gold standard and guinea became a colloquial or specialised term.
The name came from the Guinea region in West Africa, where much of the gold used to make the coins originated. Although no longer circulated, the term guinea survives in some circles, notably horse racing, and in the sale of rams, to mean an amount of one pound and one shilling (21 shillings) or one pound and five pence in decimalised currency.
The designs of the coins, as was typical of the period, are generally exactly the same as those of other denominations and, given that their values are not inscribed on the coins, the inscriptions are the same too. Thus, coins issued at the same time look virtually the same and the different denominations can only be distinguished by their composition and size.
The Guinea denominations were demonetised with the recoinage of 1816. | 
