Dollar coins have not been very popular in the United States. Silver dollars were minted intermittently from 1794 through 1935; a copper-nickel dollar of the same large size, featuring President Dwight D. Eisenhower, was minted from 1971 through 1978. Gold dollars were also minted in the 19th century. The Susan B. Anthony dollar coin was introduced in 1979; these proved to be unpopular because they were often mistaken for quarters, due to their nearly equal size, their milled edge, and their similar color. Minting of these dollars for circulation was suspended in 1980 (collectors' pieces were struck in 1981), but, as with all past U.S. coins, they remain legal tender. As the number of Anthony dollars held by the Federal Reserve and dispensed primarily to make change in postal and transit vending machines had been virtually exhausted, additional Anthony dollars were struck in 1999. In 2000, a new $1 coin, featuring Sacagawea, (the Sacagawea dollar) was introduced, which corrected some of the problems of the Anthony dollar by having a smooth edge and a gold color, without requiring changes to vending machines that accept the Anthony dollar. However, this new coin has failed to achieve the popularity of the still-existing $1 bill and is rarely used in daily transactions. The failure to simultaneously withdraw the dollar bill and weak publicity efforts have been cited by coin proponents as primary reasons for the failure of the dollar coin to gain popular support.
In which century were milled dollars minted?