Decimal Day in the United Kingdom and in Ireland was Monday 15 February 1971, the day on which each country decimalised its respective £sd currency of pounds, shillings, and pence.

Before this date the British pound sterling (symbol "£") was subdivided into 20 shillings, each of 12 (old) pence, a total of 240 pence. With decimalisation, the pound kept its old value and name, but the shilling was abolished, and the penny was revalued, such that the pound was subdivided into 100 of what were originally called "new pence" ("NP"), and later just pence ("p") when confusion was no longer likely. Each new penny was worth 2.4 old pence (abbreviated "d"). A coin of half a new penny was introduced to maintain the granularity of the old penny, but was dropped in 1984 as inflation reduced its value. An old value of 7 pounds, 10 shillings, and sixpence, typically abbreviated £7/10/6 or £7 10s. 6d. became, only, £7.521/2p. Amounts with a number of old pence which was not 0 or 6 did not convert into a round number of new pence. The Irish pound had the same £sd currency structure, and the same decimalisation was carried out.



The British decimal fifty pence coin (often shortened to 50p in writing and speech) is a unit of currency equalling one half of a pound sterling. It is a seven-sided coin formed as an equilateral-curve heptagon, or Reuleaux polygon, a curve of constant width, meaning that the diameter is constant across any bisection. Its obverse has featured the profile of Queen Elizabeth II since the coin's introduction in 1969. Four different portraits of the Queen have been used, with the latest design by Jody Clark being introduced in 2015. The second and current reverse, featuring a segment of the Royal Shield, was introduced in 2008.

As of March 2014 there were an estimated 948 million 50p coins in circulation.50p coins are legal tender for amounts up to the sum of £10 when offered in repayment of a debt; however, the coin's legal tender status is not normally relevant for everyday transactions.