The Currency of Guernsey is the Pound. Since 1921 Guernsey has been in Currency Union with the United Kingdom, but issues her own coins and banknotes.
The main unit of currency until the beginning of the Nineteenth Century was the Livre Tournois (Tour's Pound). This was divided in 20 Sols which in turn was made up of 12 deniers (as the Pound Sterling was previously divided into 20 Shillings of 12 pence each).
The rate of exchange to the Pound Sterling varied over the years; dropping from about 3 shillings (15p) in the 14th Century to around 9d (3.5p) in the 1830s.
France adopted the decimal Franc (worth 1.0125 Livres) in 1795, but the old coins continued to circulate in Guernsey for some time. Eventually the number and quality of the coins deteriorated until in 1830 it was decided to issue local coinage in the form of the Double. One double had the value of the French 'Liard' coin which was worth 3 deniers.
Guernsey wills up to the 1830s often distinguish bequests as being either in Livre Tournois or in 'Livres Sterling'. Usually this was dependent on the source of the asset involved. If it was held locally it was in Livre Tournois, or if on the mainland it would be in Sterling.
The local coins continued to circulate with the Guernsey 8 double piece being roughly equivalent both to the British penny (although actually worth slightly less) and also to the French 10 centimes piece. The size of the three coins was also similar. The same applied to the 4 double/ha'penny/5 centimes coins. There is also some evidence (from coins found in Guernsey) that the Italian 10 Centesimi coin also circulated - again this was roughly equivalent in size and value to the 8 double coin.
Banknotes were issued in 1827 and these were denominated in Sterling. In 1848 an ordinance set the rate of exchange as 2040 doubles to the Pound Sterling, although this was later rescinded. In 1870 British coins became legal tender.
Various ordinances were issued in the Nineteenth century in an attempt to control the types and values of the coins circulating. Eventually in 1902 an ordinance declared that only the copper coins of Guernsey issued after 1863, and British copper coins would be legal tender.
In effect Guernsey had two currencies circulating at the same time. However the difference in value between the Sterling penny and the Guernsey eight double was immaterial at the level of everyday use. Only with large sums did it become significant.
Banknotes were issued in Guernsey Shillings and Pounds in 1914.
After the First World War the value of the French Franc declined against the Pound Sterling which led to Guernsey adopting a Pound at par to Sterling. Reflecting previous usage the Guernsey eight double was equivalent to the UK penny for values up to 1 shilling. Above that level it was set as 21 Guernsey Shillings (2016 doubles) to the Pound Sterling. The pre war banknote designs were over-stamped "British" to distinguish them.
The 4 and 8 double coins circulated up to 1971, but the 1 and 2 double coins gradually became worthless. A Guernsey 3d coin was also minted after 1956, but there were no local versions of UK silver coins above that value. Banknotes continued to be issued
Along with the UK and Ireland, Guernsey decimalized it's currency in 1971. For the first time a full range of locally issued coins matched those in the UK. The 5p (equal to the old shilling) and 10p (two shillings) coins were actually issued in 1968 and the 50p (ten shillings) in 1969. These circulated alongside the older coins and the ten shilling note. The pound coin was issued for the first time in 1981 a few years before it's introduction in the UK. The coin designs were updated in 1985.