What Is a Mint Mark on a Coin?
A mint mark identifies on the coin itself where a coin was minted. According to the U.S. Mint, the purpose of such marks was originally to hold the mint facility responsible for the quality of the coin. The quality of the alloys would be essential in, say, a monetary standard based on precious metals, like the gold standard or a bi-metallic standard—both of which have occurred in our history.
Each coin would be struck to exacting standards for weight and metal content. When tested, if the coin's quality would fall short, a mint mark would prove the mint facility that struck the coin without question. In addition to the precious metal content, the quality of the clad planchet can be a problem from time to time, as can the quality of the strike.
From 1965 through 1967, no mint marks appeared on circulating U.S. coinage despite all three mint facilities being active at that time. The Philadelphia, Denver, and San Francisco branches were all striking coins for circulation. Mint marks would not be reinstated until 1968.
Historically, the U.S. mint marks are as follows: the Philadelphia Mint uses either no mint mark or a "P." The historic and no longer operational Charlotte, Dahlonega and New Orleans branch Mints used "C," "D," and "O" mint marks, respectively. The fabled Carson City Mint used "CC” and is notably the only U.S. mint to use more than a single letter. Still operational today, the San Francisco Mint uses the "S" mint mark. Denver has taken over the "D" mint mark from the now-defunct Dahlonega branch. The years of operation for the Dahlonega and Denver mints do not overlap and, as such, pose no threat of confusion. Finally, the West Point mint uses the now-iconic "W" mint mark.