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Coins are usually metal or alloy, or sometimes made of synthetic materials. They are usually disc shaped. Coins made of valuable metal are stored in large quantities as bullion coins. Other coins are used as money in everyday transactions, circulating alongside banknotes. Usually the highest value coin in circulation (i.e. excluding bullion coins) is worth less than the lowest-value note.
If the difference becomes significant, the issuing authority may decide to withdraw these coins from circulation, possibly issuing new equivalents with a different composition, or the public may decide to melt the coins down or hoard them