Like all collectibles, the rare coin market has its own terms and slang. The following is a brief definition and explanation
of the most frequently used coin collecting terms.
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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-D dime, for instance, is usually considered the key coin of the Mercury dime series. It is the lowest mintage coin of the set and the most expensive (in most grades). The 1919-D dime is the "condition rarity key" of the Mercury dime series, as it is the most expensive coin in top condition.
Most sets have more than one key coin. In Lincoln cents, for instance, the 1909-S V.D.B., the 1914-D, the 1922 Plain and 1955/55 Doubled Die are all considered to be key coins in most grades. In MS65RD the 1926-S is the rarest of the regular issues, so it is considered the "condition rarity key."
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."