Barrel proof

Cask strength (also known as barrel proof) is a term used in whisky-making to describe the level of alcohol-by-volume (abv) strength that is used for a whisky during its storage in a cask for maturation – typically in the range of 60–65% abv.

Most bottled whisky is diluted with water to bring its strength (i.e., its abv level) down to a level that makes it less expensive to produce and more palatable to most consumers, usually about 40% abv – a level that is the statutory maximum in some countries, and the statutory minimum in others. The degree of dilution is said to bring out different flavours of the whisky, which can affect the decisions of the producers regarding the concentration they select for the bottling of their various brands.

Whisky was served undiluted until the 20th century. Standard proof of alcohol were created around that time, and distillers began adding water to the whisky to reduce the proof.

Cask strength is not the highest proof that a whisky can be. Still-strength whisky is a higher proof, which comes out of the still at about 70% abv. Some distillers bring down the proof by adding water to the whisky prior to casking it. Further aging of the whisky in a cask will reduce the proof.

Undiluted whisky is considered cask strength. To achieve a specific alcohol proof, water is added to the whisky. The vast majority of whisky bottled by distillers is watered down to about 40% abv, some whiskies are bottled at cask strength.

Sometimes bottlers dilute even cask strength bottling, but to a lesser degree, such as to 60% abv. This avoids the need to reprint the labels – which, in some jurisdictions such as the EU, must state the exact alcohol level—due to different barrels containing whisky at different strengths.

Consumers often add water to their cask strength whiskies in order to bring out different whisky flavors. Diluting the whisky with varying amounts of water will modify the flavor profile, and some consumers like to have control over that. Some consumers add water directly to the whisky, while other consumers will add ice. The water affects the taste of the whiskey, so the selection of water is important.

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Wikipedia