A Double Eagle is a gold coin of the United States with a denomination of $20. (Its gold content of 0.9675 troy oz was worth $20 at the 1849 official price of $20.67/oz.) The coins are made from a 90% gold (0.900 fine = 21.6 kt) and 10% copper alloy. 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, D.C. 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 (when the official price of gold was changed to $35/oz by the Gold Reserve Act). 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". In 1850, the Double Eagle had the purchasing power of $1200 today. Source: Wikipedia