The term "proof" when applied to coins refers to a specialized minting process. Burnished coin blanks (planchets) are manually fed into presses with specially polished dies. The coin is struck several times so that frosted images (the detail of the coin) appear against a mirror-like background (field). A complete proof set consists of proof coins from each of the denominations minted in a given year. A silver proof set contains proof coins from each denomination made in silver. Unlike general circulation silver coins, silver proof set coins are made of 90 percent silver. Since 1968, the US Mint at San Francisco has had the responsibility for producing annual proof coin sets and silver proof coin sets. The Mint sells these proof sets to the general public in the year in which they are produced. The coins included in proof sets vary, as the array of coins produced in a year varies, and as the Mint begins and discontinues special programs, such as Prestige Proof Sets, which contain a commemorative dollar or half dollar for that year. In 1999, the Mint began issuing a series of 50 commemorative quarters, one for each state. Five such quarters are issued each year, and proof sets for these quarters have been issued as well. They may be purchased as a separate set, or in conjunction with the regular annual proof set. For those who wish to collect proof sets of recent vintage, several internet sites list the proof sets available for each year from 1936 on. Since the Mint only sells proof sets for the current year, collectors obtain proof sets for prior years from dealers or private sellers. | 
