The eagle was a base-unit of denomination issued only for gold coinage by the United States Mint.
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base-units of denomination used for circulating coinage in the United States prior to 1933, the year when gold was withdrawn from circulation.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, the gold half-eagle, the eagle, and the double-eagle coins.
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 comprised a majority of the alloy used in that coin.
In this regard the United States followed long-standing British and 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.
During the 1850s following the discovery of gold in California, an additional fifth base-unit of denomination was proposed, called a "union" whose value would have been 10 eagles or, equivalently, 100 dollars.
Although patterns of half-union gold coins were minted and presented for inspection to Congress, Congress did not pass a law that authorized the minting of union-denominated and half-union-denominated coins for circulation because the double-eagle coin was viewed as a substantial sum of money for the general public at the time and a half-union or union coin would primarily have circulated only among banks and other financial institutions.
They were also perceived as being unwieldy, weighing in at around 2½ ounces. Despite this absence of a half-union coin for general circulation, a single issue of a gold commemorative $50 coin was minted in 1915.
Although technically still legal tender, the numismatic value vastly exceeds their face value. They are generally held either by coin collectors, investors, or museums.
Quarter eagles were issued for circulation by the United States Mint from 1796 until 1929; half eagles from 1795 until 1929; eagles from 1795 to 1933; and double eagles from 1850 to 1933, although for each of these ranges of years there were occasional gaps in production. The diameter of quarter eagles was 17 mm; of half eagles 21 mm; of eagles 27 mm; and of double eagles 34 mm.
Originally the purity of all circulating gold coins in the United States was 22 karats (11 parts gold to 1 part alloy). The weight of quarter eagles was 67.5 troy grains (4.37 g); of half eagles 135 troy grains (8.75 g); of eagles 270 troy grains (17.5 g).
In 1834, the mint value of gold to silver of 15:1 was changed to 16:1 and the standards for both gold and silver coins changed because at the old ratio it was profitable to export and melt U.S gold coins.
A small additional change in the fineness of the gold was made in 1837 before any eagles could be minted. The new standard for the eagle was 258 troy grains (16.718 g) of .900 fine gold, with other coins proportionately sized. This new standard would be used for circulating gold coins until circulation was halted in 1933.
Images courtesy of Heritage Numismatic Auctions and Superior Stamp & Coin
Source: Wikipedia