The official mint set for a particular year contains at least one uncirculated example of each coin denomination from each mint that produced the denomination during that year. Although mint set coins have not been circulated, they are not proof coins, and no particular care was taken in their production. There have been some variations in the composition and appearance of mint sets over the years. From 1947 to 1958, the US Mint included two samples of each denomination from each mint, except during 1950, when no mint sets were issued. The coins in these early mint sets were mounted in cardboard holders. In 1959, the US Mint began issuing mint sets in plastic envelopes, which better preserved the coins. The number of samples of each coin was reduced from two to one. Official mint sets were not released in 1982 and 1983. During the years 1965 through 1967, the US Mint issued a series of special mint sets. These coins in these sets were closer to proof coin quality, and were packaged in special plastic cases. In 1976, a special three piece bicentennial set was released in addition to the regular issue set. The three piece set contained the Bicentennial quarter, half dollar, and dollar made with 40 percent silver. The regular set for 1976 also contains these coins, but they are made with a combination of nickel and copper.