The British decimal fifty pence (50p) coin - often pronounced 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 when the 50p joined the 5p (shilling) and 10p (florin) coins in circulation, leaving only the three copper coins 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).
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.
The 50p coin was reduced in size in 1997 and the older (large) coins were removed from circulation; they were demonetised in 1998. The design of the new (small) type remained unchanged.
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).
1997 was the last year when Raphael David Maklouf's effigy of Queen Elizabeth II (the Third Portrait) was used on a coin of this denomination.