Many new collectors eventually stumble upon a very important term that comes up a lot when building coin collections – especially those involving the completion of date-and-mintmark runs. That term is “key coin,” and it usually refers to the coin (or coins, plural) that are the rarest and most valuable of the set. In terms of rarity, mintage is frequently the determinant factor. However, in some cases a key coin might not have the lowest mintage per se but rather claims the fewest number of known survivors among all issues in its given series. “Key” could also refer to the toughest coin to find at a certain grade level, such as MS65 or MS67; “conditional key” is a term whose definition is often of great pertinence to PCGS Set Registry members.
As PCGS further describes the term key coin, it’s “[t]he 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 [as] a ‘key’ coin. The terms ‘key to the set’ or ‘key to the series’ are also used as synonyms for ‘key coin.’
Deciding how and when to buy coins like keys often comes down to your budget and preferred collecting strategy. Most certainly cost is a major factor for collectors when it comes to forking over the cash (or plastic) to purchase the necessary keys for completing a set. But that’s not the only factor that influences the timing of such a purchase. So, too, does the method in which you wish to build your set. Many collectors buy the keys and other scarce coins first to get the big purchases out of the way. Others take the opposite approach, buying the lesser-expensive coins first to physically build the biggest portion of their set, saving the acquisition of key coins for last. There is no “correct” way to build a coin collection and surely no better method for deciding when to buy key coins for your sets.