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Seven sides, a bimetallic ring, a scalloped edge — denomination by feel.
Also known as: Seven-sided coin, Coin with two metals, Shape of a coin
Coins differ in shape as well as size for the same reason notes carry raised dots: so they can be told apart without being seen, and so a vending mechanism can reject the wrong one.
Circulating coins are distinguished by diameter, thickness, mass, metal, edge treatment and outline, several currencies using non-circular shapes for particular denominations.
Contexts: History
The British fifty and twenty pence are Reuleaux heptagons — seven-sided figures of constant width — so they roll through a coin mechanism like a circle while being unmistakable by touch.
United Kingdom
Bimetallic construction, a ring of one metal round a core of another, is both a tactile and a mechanical distinction: it is hard to counterfeit and easy for a machine to verify by electrical signature.
A metal disc, sometimes with seven curved sides or a scalloped rim, and sometimes with a ring of one colour of metal around a core of another.