Une pièce de monnaie est un morceau de matériau solide, habituellement un métal ou un métalloïde, ayant la forme d’un disque, et la plupart du temps délivré par le gouvernement. Les pièces de monnaie sont utilisées comme argent dans des transactions de toutes sortes, à partir des transactions de tous les jours jusqu’au stockage du vaste nombre de pièces précieuses.
Bi-metallic coins are coins consisting of more than one metal or alloy, generally arranged with an outer ring around a contrasting center. Common circulating examples include the €1, €2, British £2, Canadian $2, South African R5, Turkish 1 lira, IDR 1K, Hong Kong $10 and all Mexican coins of $1 or higher denomination.
Bi-metallic coins have been issued for a long time, with examples known dating from the 17th century, while the Roman Empire issued special occasion, large medallions with a center of bronze or copper and an outer ring of orichalcum, starting with the reign of Hadrian. The silver-center cent pattern produced by the United States in 1792 is another example. In the 1830s and 1840s, British medalist Joseph Moore produced large numbers of bi-metallic "penny model" and less common "halfpenny model" tokens, as a proposal to replace the relatively large penny and halfpenny coins. Though not legal tender, Moore's tokens were circulated widely and accepted at face value by many merchants. Despite their popularity, the Royal Mint rejected the proposal, and did not reduce the size of the penny and halfpenny until decimalization.