According to Federal regulations, for an American whiskey to be labeled as bourbon it must be made from a mash containing between 51% and 79% corn. If the corn content is higher, the product must be designated as corn whiskey. Bourbon is a straight whiskey and, according to the law, must be distilled at 160 proof (80% alcohol) or less and must be aged a minimum of two years in new charred oak barrels. As a practical matter, though, most bourbon is aged at least four years and often longer. Since it is a straight whiskey, no blending is permitted and there are no additives, with the exception of water to reduce the proof.
Most bourbons are marketed as 80-proof products, but some, particularly the newer boutique, small-batch, single barrel and barrel proof products are much higher in alcohol content.
By law, bourbon can be distilled anywhere in the U.S., but the vast majority of it is produced in Kentucky, where it must be distilled and warehoused for at least one year in order to carry the "Kentucky Bourbon" designation on the label.