Proof coinage means special early samples of a coin issue, historically made for checking the dies and for archival purposes, but nowadays often struck in greater numbers specially for coin collectors (numismatists). Many countries now issue them.
Preparation of a proof striking usually involved polishing of the dies. They can usually be distinguished from normal circulation coins by their sharper rims and design, as well as much smoother fields.
Modern U.S. proof coins are often treated with chemicals to make certain parts of the design take on a frosted appearance, and the fields taking on a mirror finish. Several other methods have been used in the past to achieve this effect, including sand blasting the dies, and matte proofs. Proof coins of the early 1800s even appear to be scratched, but it was part of the production process.
Most proof coins are double struck. This does not normally result in doubling that is readily observable, but does result in the devices being struck fully.
The U.S. largely stopped striking proof coins in 1916, although a few later specimens exist. Beginning in 1936, the U.S. Mint began producing proof sets. Sets struck from 1936–42 and, when resumed, from 1950–72 include the cent, nickel, dime, quarter, and half dollar (From 1965 to 1967, the production of proof sets were suspended and Special Mint Sets were made in their place. They were made due to a coin shortage. They were made at the San Francisco Assay Office but beared no "S" mintmark.) From 1973 through 1981 the dollar was also included, and also from 2000 on. Regular proof sets from 1982 to 1998 contain the cent through the half dollar.
Other sets, called Prestige Proof sets, also contain commemorative coins. These sets were sold from 1983 to 1997 at an additional premium. As Legacy Proof sets, the practice was resumed in 2005. Beginning in 1999, proof sets also contain five different Statehood Quarters. The 2004–05 series also contain the two Lewis and Clark nickels.
Since 1992 the mint has struck proof sets in both silver and base metal. U.S. commemorative and bullion platinum, gold, and silver coins are also often issued in both uncirculated and proof types, sometimes with different Mint marks.
In most years since 1947 the U.S. mint has also produced "mint sets", and because of the terms used there is some confusion over the difference between these and proof sets. These are uncirculated coins that have been specially packaged, and (unless a scarce coin is included) are generally neither as expensive nor as valuable as proofs. Some U.S. mints also sell annual "souvenir sets" from their production runs and individual dealers have made unofficial "year sets." The latter have no value beyond their individual coins. Members of the public should be careful to understand what products they are being offered.