The eagle is a base-unit of denomination issued only for gold coinage by the United States Mint. It has been obsolete as a circulating denomination since 1933. The eagle was the largest of the four main decimal base-units of denomination used for circulating coinage in the United States prior to 1933, the year when gold was withdrawn from circulation. These four main base-units of denomination were the cent, the dime, the dollar, and the eagle, where a dime is 10 cents, a dollar is 10 dimes, and an eagle is 10 dollars. The eagle base-unit of denomination served as the basis of the gold quarter-eagle (US$2.50), the gold half-eagle (US$5), the eagle (US$10), and the double-eagle coins (US$20).
With the exceptions of the gold dollar coin, the gold three-dollar coin, the three-cent nickel, and the five-cent nickel, the unit of denomination of coinage prior to 1933 was conceptually linked to the precious or semi-precious metal that constituted a majority of the alloy used in that coin. In this regard the United States followed long-standing European practice of different base-unit denominations for different precious and semi-precious metals. In the United States, the cent was the base-unit of denomination in copper. The dime and dollar were the base-units of denomination in silver. The eagle was the base-unit of denomination in gold.
The United States’ circulating eagle denomination from the late 18th century to first third of the 20th century should not be confused with the American Eagle bullion coins which are manufactured from silver (since 1986), gold (since 1986) or platinum (since 1997).
Article Source: en.wikipedia.org