True American bourbon must be made from a mash bill, or mix of grains, which in turn must contain at least 51% corn. The rest is usually rye or wheat with a small amount of malted barley, but it can also include rice or oats, for example. True bourbon must also be aged in new charred oak containers, although those containers don't have to be barrels specifically, at least according to the U.S. Alcohol and Tobacco Trade and Tax Bureau. Finally, bourbon must be distilled in the United States to no more than 160 proof (and bottled at a minimum of 80 proof). Finally, no flavoring or color can be added to bourbon.