American whiskey is a type of distilled alcoholic beverage made from fermented grain mash. Various grains are used for different varieties, including corn, rye, barley, and wheat. Whisky is usually aged in wooden casks, generally made of charred white oak. Whisky is a strictly regulated spirit worldwide with many classes and types. The typical unifying characteristics of the different classes and types are the fermentation of grains, distillation, and aging in wooden barrels.