Coin history
How to read a U.S. coin's mint mark
A mint mark is a small letter on a U.S. coin that shows which mint struck it. P is Philadelphia, D is Denver, S is San Francisco, W is West Point and CC is Carson City.
Many Philadelphia coins carry no mint mark at all, so a coin with no letter is often a Philadelphia coin. The mark's location varies by series, so check where it appears on the type you have.
Mint marks matter to collectors because some years were struck at only one mint. A coin from a low-output mint can be scarcer than the same date from another mint.