A mint mark, put simply, is a small letter or insignia to indicate where a coin was made. In American numismatics, mint marks were not used until 1838 when the first branch mint facilities were opened. Before 1838 all U.S. coins were struck in Philadelphia; there was no need to differentiate between various facilities. This would change as America expanded and precious metals were discovered in faraway remote places.
In the late 18th and early 19th centuries, all United States coins were struck in Philadelphia and made from bullion deposited with the Mint. This bullion was sourced from banks and merchants in the form of foreign coins, ingots or even everyday objects like silverware. A depositor would submit raw metal to the Mint and, once it was assayed, receive gold or silver coins in exchange. This was the primary source of metal for many years, but eventually precious metals were being extracted from the earth in the south.
In the 1830s, gold was discovered in North Carolina and Georgia. At the time transporting the valuable metal to Philadelphia would have been difficult and expensive; therefore mint facilities were opened in Charlotte NC and Dahlonega GA. These branch mints were small but served the important function of converting this raw metal into money. To differentiate coins struck at these facilities from the “mother mint,” the mint marks C and D were used for Charlotte and Dahlonega respectively. That same year New Orleans received a new Mint too; coins from that location featured the O mint mark.
San Francisco was the next to receive a Mint, thanks to the massive amount of yellow metal extracted during the California Gold Rush. Coins from San Francisco feature an S mint mark. Similarly, the Carson City Mint was opened in response to the Comstock Lode—one of the largest quantities of silver discovered in the United States. Just like in Charlotte and Dahlonega, it was decided that converting the metal into coinage locally was far preferable to hauling the bullion long distances.
The 20th century featured the opening of the Denver and West Point Mints, both of which are still in operation today. They use the D and W mint marks respectively. The Denver location produces coins for circulation, whereas West Point primary strikes bullion coins. San Francisco is still in operation, but it is used mostly for proof and collector-edition pieces. The Philadelphia Mint remains the dominant mint; it produces the majority of coins for circulation and services the large eastern population base.
Coins produced at the Philadelphia Mint used to lack a mint mark—as it was the primary mint—but starting in the 1940s certain Philadelphia coins began displaying a P mint mark.