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 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 commemorates the 250th anniversary of the Royal Academy.
The Royal Mint says:
"For 250 years, the Royal Academy of Arts (RA) has been at the forefront of championing the arts in Britain through education and exhibition. 2018 sees the RA250 celebrations - a significant moment where an iconic British institution reaches 250 years of supporting and promoting the arts. To mark this occasion, the RA and The Royal Mint have collaborated on a £5 silver Proof coin.
Designed by Sir David Chipperfield RA (RA is the post-nominal for Royal Academician), who also led the redevelopment of the RA by linking Burlington House and Burlington Gardens to create more space for art, education and debates, the coin features the façade of Burlington House, the familiar entrance for millions of art lovers, artists and architects." | 
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