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 celebrates the Platinum Wedding (or 70th anniversary of the marriage) of Queen Elizabeth II to Prince Philip, Duke of Edinburgh.
The reverse was designed by John Bergdahl, and the obverse by Etienne Millner. The Royal Mint says about the design:
"To have fallen in love completely and unreservedly makes all one’s personal troubles and even the world’s seem small and petty." Written by Philip in 1946, these words demonstrate the strength of his love for Princess Elizabeth, which inspired him when designing her engagement ring. Diamonds from his mother’s tiara were set in platinum and The Queen still wears the ring to this day.
Etienne Millner’s combined portrait graces the obverse, while John Bergdahl’s double shield design features the heraldic lineage of the two families that became entwined when the couple married.
Another crown coin issued at the same time has the same obverse with a reverse featuring the royal couple on horseback. | 
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