Almost every United States coinage series has certain dates that are difficult to obtain for one reason or another. Some dates have exceptionally low mintages.
Coins of these dates are key coins that are fundamentally rare. That is, they are rare in all conditions because only a limited number was struck. Other coins are rare in uncirculated or higher uncirculated grades. These coins are called condition rarities and are often key coins as well. Then there are some coins that are available in all conditions, but since the series is so popular, the scarcer dates become key coins even though in absolute numbers they are not rare. Finally, some coins are actually mint errors that become rare because of collector demand. This category includes doubled dies and re-punched dates and mintmarks.
Some examples can be illustrative. The 1794 Silver Dollar had a mintage of 1,758. No matter how many millions of collectors there are in the world, only that number of collectors can obtain an example of this coin. In fact, the actual number is much less because this is only a mintage number and does not account for the actual survival rate for this issue. In addition, many examples are impounded in museums and national collections making them unobtainable by the general public, even for a price. Certainly the 1794 dollar is a key date coin. Continuing with dollars, an 1872-S Seated Liberty dollar can be obtained in circulated condition for three to five hundred dollars. In mint state 65 it lists for $131,250, clearly a condition rarity and a key coin in higher mint state grades. The High Relief, Roman Numeral Double Eagles of 1907 had a mintage of 12,367, not a tiny mintage by any means. Yet because of the number of collectors who desire an example, the coin is always in demand in all conditions and especially in mint state. Finally, there are thousands of different doubled dies in many series, but there is none more famous than the Lincoln 1955 Doubled Die Obverse coin. This coin has become a key date although many other doubled dies, re-punched dates and mintmarks go unnoticed except by specialists.
The following list of key date coins does not include proofs unless they are "proof only" for the date. For example, the 1895 Morgan Dollar is a "proof only" coin since no business strikes were released for that year. Collectors would have to obtain a proof coin to have a complete date set that included 1895.
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