Key Coin: A coin that is among the toughest and Most Expensive to obtain within a series. In the Lincoln Cent series, for example, the 1909-S VDB would be considered the key, as would the 1914-D and often the 1922-Plain.
A coin somewhat lesser in stature but still among the tougher in a series to collect is called a semi-key; the 1909-S and 1931-S fulfill this role within the Lincoln Cent series for many collectors.
The Concept of “Key" may also apply to the Type or Purpose of the coin collection, rather than the individual coin. For example, if a collector is putting together a collection of Walking Liberty Half Dollars, without regard to the Condition of the coins, then the “keys" are the 1921, 1921-D, and 1916-S.
But, if the Collector is putting together the same set, in Gem BU Condition, the 1919-D and the 1921-S would be considered the “keys" and the 1916-S likely would not, because the 1916-S either not available at all, or is plentiful. Also Key Date may be a replaceable term.
Another example, using Lincoln Cents: The "key dates" for the Lincoln Pennies, from 1909 -1958 (wheat cents) are: 1909s, 1909s VDB, 1914 D, 1922, and the 1931 S.
Certain Sellers use the term “Key Date" to generate interest in a coin, whose date may be “Key", however, the Condition of the coin is usually less than desirable. Pay Attention!
