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Whiskies do not mature in the bottle, only in the cask, so the "age" of a whisky is only the time between distillation and bottling. This reflects how much the cask has interacted with the whisky, changing its chemical makeup and taste. Whiskies that have been bottled for many years may have a rarity value, but are not "older" and not necessarily "better" than a more recent whisky that matured in wood for a similar time. After a decade or two, additional aging in a barrel does not necessarily improve a whisky.[24]

While aging in wooden casks, especially American oak and French oak casks, whisky undergoes six processes that contribute to its final flavour: extraction, evaporation, oxidation, concentration, filtration, and colouration.[25] Extraction in particular results in whisky acquiring a number of compounds, including aldehydes and acids such as vanillin, vanillic acid, and syringaldehyde.[26] Distillers will sometimes age their whiskey in barrels previously used to age other spirits, such as rum or sherry, to impart particular flavours.