Bourbon whiskey is a type of American whiskey: a barrel-aged distilled spirit. The typical grain mixture for bourbon, known as the mash bill, is a minimum of 51% corn, with the remainder being rye, wheat, and/or malted barley. The spirit is aged in newly charred American oak barrels from which it gains color and flavor from the caramelized sugars in the charred wood. Bourbons gain more color and flavor the longer they mature. Maturity, not a particular age, is the goal.