Cask strength is not the highest proof for a whisky. Still-strength whisky is typically a higher proof. Whisky produced by a
pot still increases in strength with each distillation and is typically distilled to about 70% ABV, and
column stills are capable of producing much higher proof levels. Most distillers reduce the proof by adding water to the whisky prior to casking it. The proof level tends to change somewhat during the aging process, depending on storage conditions.
Scotch whisky is typically aged in used barrels, and due to the relatively cool climate in Scotland, the proof level typically stays the same or goes down during maturation. In contrast, American
bourbon whiskey is produced using new barrels. Storage conditions in Kentucky and Tennessee, where nearly all of it is produced, allow the proof levels to rise during aging. In the United States, the use of various terms, including "barrel proof", on product labels is regulated by
truth in labelling requirements. Under ruling 79-9 of the
U.S. Bureau of Alcohol, Tobacco and Firearms, a whisky can only be called "barrel proof" if the bottling proof is not more than 1% ABV (2 degrees
U.S. proof) lower than when the barrels were dumped at the end of the aging period. The ruling also covers the use of several other phrases describing high-proof whiskies. The phrases "original proof", "original barrel proof", and "entry proof" are restricted to "indicate that the proof of the spirits entered into the barrel and the proof of the bottled spirits are the same". ==Drinking==