At the time of decimalisation in 1971, the United Kingdom Crown coin (equal to 5 shillings) was redenominated as a 25p coin and pieces struck prior to 1990 - all the way back to 1818 - continue to be legal tender for that amount. The old denomination had been a regular circulating coin in the 19th century, but in the 20th century it became more of a commemorative denomination.
The Royal Mint continued issuing commemorative coins in the same format - using the traditional coin dimensions, but in copper-nickel. These were initially also denominated as 25 pence but the "Crown" was changed in 1990 to a commemorative £5 (five pound) coin, having the same dimensions and weight but with a face value twenty times as great.
The denomination is used to mark special occasions, usually royal in theme, rather than for use in general circulation. Although not circulating, these coins are legal tender.
This commemorative £5 Crown-sized coin was issued as part of a six-coin set of similar coins commemorating the Centenary of the First World War; the set itself is part of a series of sets issued over several years. The coin shows one of the first air planes used in combat and commemorates 100 years since the founding of the Royal Air Force.
The Royal Air Force (RAF) is the United Kingdom's aerial warfare force. It was formed towards the end of the First World War on 1 April 1918. Following the Allied victory over the Central Powers in 1918, the RAF emerged as the largest air force in the world at the time. Since its formation, the RAF has taken a significant role in British military history. In particular, it played a large part in the Second World War where it fought its most famous campaign, the Battle of Britain.
The coin was issued in Proof FDC, in sterling silver and in a gold, both in sets only and not issued individually. | 
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