A Double Eagle is a gold coin of the United States with a denomination of $20. Although the "eagle"-based nomenclature for gold U.S. coinage is often assumed to be a nickname, the "eagle", "half-eagle" and "quarter-eagle" were specifically given these names in the Act of Congress that originally authorized them ("An Act establishing a Mint, and regulating Coins of the United States", section 9, April 2, 1792).
Likewise, the Double Eagle was specifically created as such by name ("An Act to authorize the Coinage of Gold Dollars and Double Eagles", title and section 1, March 3, 1849).
The first double eagle was minted in 1849, coinciding with the California Gold Rush. In that year, the mint produced two pieces in proof.
The first resides in the Smithsonian Institution in Washington DC.
The second was presented to then Treasury Secretary William M. Meredith and was later sold as part of his estate - the present location of this coin remains unknown.
In 1850 regular production began and continued until 1933. Prior to 1850, eagles with a denomination of $10 were the largest denomination of US coin. $10 eagles were produced beginning in 1795, just two years after the first U.S. mint opened.
Since the $20 gold piece had twice the value of the eagle, these coins were designated "double eagles".
Regular issue
Regular issue double eagles come in two major types and six minor varieties as follows:
Liberty Head (Coronet) 1849–1907
Liberty Head, no motto, value "Twenty D." 1849–1866
Liberty Head, with motto, value "Twenty D." 1866–1876
Liberty Head, with motto, value "Twenty Dollars" 1877–1907
Saint Gaudens 1907–1933
Saint Gaudens, High Relief, Roman Numerals, no motto 1907
Saint Gaudens, Low Relief, Arabic Numerals, no motto 1907–1908
Saint Gaudens, Low Relief, Arabic Numerals, with motto 1908–1933
Images courtesy of Heritage Numismatic Auctions
Source: Wikipedia