The 2006 United States Mint Proof Set includes five quarters commemorating the following states: Nevada, Nebraska, Colorado, North Dakota, and South Dakota. These quarters were part of the 50 State Quarters Program, which began in 1999 and ended in 2008. The coins in the set are struck in proof quality, which means they have a frosted image and a mirror-like background. In addition to the state quarters, the set also includes a proof version of the Lincoln cent, Jefferson nickel, Roosevelt dime, Kennedy half dollar, and Sacagawea dollar. The 2006 United States Mint Proof Set was released on March 13, 2006, and had a limited mintage of 1,092,406 sets. The set was sold by the United States Mint for a retail price of $22.95. Today, the set may be worth more than its original retail price, depending on its condition and whether it has been certified by a third-party grading service.