The British fifty pence (50p) coin - often pronounced fifty pee - is a unit of currency equaling 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 when the 50p joined the 5p (shilling) and 10p (florin) coins in circulation, leaving only the three copper coins (1/2p, 1p and 2p) to be introduced on 15 February 1971 to complete the new series of decimal coins; unlike other coin types at the time though, the 50 pence coin was not made equivalent to a pre-decimal coin; it was the same as a crown in value but not in appearance or size.
With the introduction of smaller 5p and 10p coins in 1990 and 1992 respectively, the 50p became the largest coin in circulation. In October 1994 the Government announced a further review of the United Kingdom coinage. The results revealed a requirement for a smaller 50p coin, which was duly introduced on 1 September 1997; it retained the design by Christopher Ironside. The large fifty pence coins were demonetised in 1998.
All circulation coins for 1997 were minted to the new standard. However, the uncirculated and proof mint sets issued this year had both the old (this) and the new 50 pence pieces (109,557 brilliant uncirculated plus 48,761 proofs in the standard set and 31,987 in a red leather case).
Additionally, a small silver set was issued with both sizes in silver, listed in Spink with mintage of 10,304 (not in Krause).