A mint mark is a small letter (D and S being the most commonly encountered) somewhere on the coin. A mint mark tells you where the coin was made. Many coins also have the designer's initials hidden somewhere in the design. This is not the mint mark. The mint mark is almost always directly on the field and not usually part of any other device.
Because many American collectors collect coins by date and mint mark, the presence of a mint mark can (and often does) change the value of a coin considerably. This is because it can affect the rarity of the coin. For example, in the case of 1894 dimes, having the 'S' mint mark adds tens of thousands of dollars to the value of the coin as only 24 were minted.
Many American coins lack a mint mark. That is because in the beginning, there was only one mint, Philadelphia. So any coin lacking a mint mark is from the Philadelphia mint (or extremely recently, it might also be from West Point, but there is no way to tell.)
The following mint marks exist:
The following list provides a hint as to where to look for the mint mark. Any coin not listed was probably only made in Philadelphia and thus lacks a mint mark.
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