Uh, a small "S" !
San Francisco struck circulating coins up to 1955 and then again sporadically from 1968 to 1975. Since 1968 all proof coins except 1999 dollars have been made there and carry an "S" mint mark.
The mint mark position on all Mercury dimes is to the right of the E in the word ONE on the reverse of the coin. No mint mark = Philadelphia D = Denver S = San Francisco
Minted in San Francisco
A mint mark is a smaller letter or letters representing the mint facility the coin was struck and are found in various places on the coin.
1879 would make it a Morgan dollar, so the mint mark is located on the reverse (tails) side, near the bottom, just above the letters "do" in the word "dollar." No mint mark means it's from Philadelphia, S is for San Francisco, O is New Orleans, and CC is Carson City.
Look on the back above the word ONE in ONE DOLLAR No mint mark - from Philadelphia S - San Francisco O - New Orleans CC - Carson City
The mint mark position on all "Mercury" dimes is next to the E in ONE on the back. No mint mark = Philadelphia S = San Francisco D = Denver
From 1946 to 1964, the mint mark position was at the bottom left of the torch on the back of the coin. No mint mark = Phila. S = San Francisco D = Denver
The mint mark position on all Lincoln cents is the same, under the date. Possibilities are:No mint mark = Philadelphia (cents do not use the "P" mint mark)D = DenverS = San Francisco
If your coin has a "S" mint mark on it means that it was made at the San Francisco mint in California.
The "S" Mintmark means it was made at the San Francisco Mint
You can determine if a 1896 barber quarter is from New Orleans or San Francisco by looking at the mint mark located on the reverse side below the eagle's tail feathers. If it has an "O" mint mark, it was minted in New Orleans. If it has an "S" mint mark, it was minted in San Francisco.
It's a mint mark indicating where the coin was made: > No mint mark = Philadelphia > D = Denver > S = San Francisco