On December 1, 1997, the 50 States Commemorative Coin Program Act was signed into law by President Clinton. Two years later, in 1999, the U.S. Mint began producing the first commemorative state quarters.
In 1999, the first year of the State Quarters Program, five states were represented: Delaware, Pennsylvania, New Jersey, Georgia, and Connecticut.
Like all coins in the United States, state quarters are produced in three specific branches of the U.S. Mint: Denver, Philadelphia, and San Francisco. The exact mint branch where a coin is produced is indicated by the mint mark found just below the year on the coin face.
The code for coins produced in Denver is "D," whereas Philadelphia is "P" and San Francisco is "S." Most state quarters, like all U.S. coins, are produced in Denver and Philadelphia.
San Francisco Coins
State quarters that are produced in San Francisco, marked with an S, are considered "proofs" or collectors' editions. They are not meant for general circulation and are available only by direct purchase from the U.S. Mint.
In terms of value, there is no difference between state quarters with P and D mint markings.
Beyond the States
When Hawaii was commemorated in 2008, the State Quarters Program completed its initial goal of representing all 50 U.S. states. In 2009, though, the program expanded to include Washington D.C. and U.S. Territories.