In this regard the United States followed long-standing European practice of different base-unit denominations for different precious and semi-precious metals.
The United States' circulating eagle denomination from the late 18th century through to the first third of the 20th century should not be confused with the American Eagle bullion coins which are manufactured from silver or gold (since 1986), platinum (since 1997), or palladium (since 2017).
Except for the double eagle, the diameters of all these denominations were decreased over time.
[4] With the Coinage Act of 1837, passed on January 18, 1837, a small change in the fineness of the gold (increased to exactly .900 fine) was made, and the alloy (now 10% of the coin's weight) was again legally defined as silver and copper, with silver capped at no more than half.
[5] (i.e. 5% of total coin weight) The new standard for the $10 eagle was 258 grains (16.7 g) of .900 fine gold, giving pure gold content of 0.48375 troy ounces (15.046 g), with other coins proportionately sized.
In 2000 a unique eagle, the Library of Congress bimetallic eagle, was issued commemorating the Library of Congress; it consisted of equal weights of an approximately 1⁄4 troy ounce (0.27 oz; 7.8 g) .9995 fine platinum core and a .900 fine gold outer ring.