Bourbon is a type of American whiskey predominantly made in the southern United States. It is made of a blend of different grains, with the bulk of the grain alcohol coming from corn, and grains such as malted barley, rye, or wheat making up the rest. Once distilled, the high-proof alcohol is aged in white oak barrels for at least two years, though often much longer, before being bottled. The bottled alcohol is usually altered so that it is 80 proof, in order to allow it to be sold in states which have a proof limit on the sale of spirits. Higher proof bourbons, often as high as 125 proof, are sometimes bottled, in which case they are usually referred to as cask strength as they are bottled at the proof at which they come out of the cask. Cask strength bourbon tends to be available only from higher-quality manufacturers and often fetches a premium price.