The British fifty pence (50p) coin - often pronounced as "fifty pee" - 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. This shape, which was revolutionary at the time, made it easily distinguishable from round coins both by feel and by sight, while its constant breadth allowed it to roll in vending machines.
The denomination was introduced in October 1969 with a large version of the coin; it was reduced in size in 1997, with the older coins being demonetised in 1998. The design of the new (smaller) type remained unchanged at that time, retaining Christopher Ironside's Britannia reverse. In 2008 though the reverse was changed to Matthew Dent's design; in a world-first concept, the designs for the 1p, 2p, 5p, 10p, 20p and 50p coins depict sections of the Royal Shield that form the whole shield when placed together (the shield in its entirety being featured on the Royal Arms type of £1 coin - which was later discontinued).
The denomination has also been used extensively to issue one-year types of commemorative coins.
Twenty pence and fifty pence coins are legal tender only up to the sum of £10; this means that it is permissible to refuse payment of sums greater than this amount in 20p and 50p coins in order to settle a debt. | 
