The US Mint has been producing Proof Sets since 1936, except for several years during WWII and the post-war period. Proof Sets were produced at the Philadelphia Mint prior to 1968 and at the San Francisco Mint beginning in 1968. Proof coins are favorites of collectors because of their mirror-like appearance, which is due to the way the coins are produced. Coin blanks are highly polished and cleaned and then fed into the press for multiple strikings. The resulting proof coin has high relief details and a cameo mirrored surface. It's no wonder that Proof Sets are prized by collectors.