Key date

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In coin collecting, a key date refers to a date (or date and mint mark combination) of a given coin series or set that is harder to obtain than other dates in the series. The next level of difficult to obtain coins in series are often referred to as semi-key dates or simply semi-keys.

For example, the 1909-S VDB is the key date in the Lincoln cent series. The 1914-D and 1931-S are considered semi-keys. In the United Kingdom £1 coins from 1988 are considered key date coins.

Professional and avid coin collectors will often not simply collect coins, but will specialize on a specific types of coin and then attempt to collect every coin in the series, i.e. one from every year that type of coin was minted with all variations, most notably mint mark, but can also be material; the coin was minted out of (some years, the same coin might be made out of two or even three different metal combinations, such as steel, nickel, iron, gold, bronze or silver) and even artistic differences or errors on the die that struck the coins and at the highest quality that can be found.

Often, locating a "key date" coin for a set is what stands between a complete set and just a collection - once completed - the entire set's value increases significantly.