Bourbon is an American form of whiskey named for Bourbon County, Kentucky. By United States law, it consists of at least 51% corn — typically about 70% — with the remainder being wheat and/or rye, and malted barley. It is distilled to no more than 160 (U.S.) proof, and aged in new charred oak barrels for at least two years. The two years maturation process is not a legal requirement for a whiskey to be called "bourbon," but it is a legal requirement for "straight bourbon." However, in practice, most bourbon whiskeys are aged for at least four years.
Bourbon must be put into the barrels at no more than 125 U.S. proof. After aging it is diluted with water and bottled. Bottling proof for whiskey must be at least 80 proof (40% abv) and most whiskey is sold at 80 proof. Other common proofs are 86, 90, 94, 100 and 107, and whiskeys of up to 142 proof have been sold. Some higher proof bottlings are "barrel proof."
Bourbon can legally be made anywhere in the United States where it is legal to distill spirits. Legitimate production is not restricted to Kentucky, although currently all but a few brands are made there, and the drink is associated strongly with that commonwealth. Illinois once produced nearly as much bourbon whiskey as Kentucky, and bourbon continues to be made in Virginia. In the past bourbon has been made in Pennsylvania, Indiana, Ohio, Tennessee, Missouri and Kansas.