key coin

The major, or most important, coin in a particular series. The “key” coin is usually the lowest-mintage coin and/or the most expensive coin in a particular set. The 1916 10 centavos, for instance, is usually considered the key coin of the 10 centavos series. It is the lowest mintage coin of the set and the most expensive (in most grades). The 1937 ABC peso is another good example.

At times any scarce or rare coin is referred to a “key” coin. The terms “key to the set” or “key to the series” are also used as synonyms for “key coin.”

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