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Whiskey from WikiPedia:

Whisky or whiskey refers to a broad category of alcoholic beverages that are distilled from fermented grain mash. Different grains are used for different varieties, including barley, malted barley, rye, malted rye, wheat, and maize (corn). Most whiskies are aged in wooden casks (generally oak), the exception being some corn whiskeys.

Whisky is a strictly regulated spirit worldwide with multiple competing denominations of origin and multiple classes and types. The unifying characteristics of the different classes and types are the fermentation of grain or corn, and the practice of distilling the spirit down to a minimum of 90% alcohol for grain, and 80% alcohol for corn, prior to adding water, so as to retain some of the flavor of the grain used to make the spirit and prevent it from being classified as grain neutral spirits or vodka. Whisky gains as much as 60% of its flavor from the type of cask used in its aging process and therefore further classification takes place based upon the type of wood used and the amount of charring or toasting done to the wood. Bourbon whiskey for example is legally required to be aged in charred new oak barrels, whereas quality Scotch whiskies often used the partially spent barrels from Bourbon production to induce a slower maturation time, adding additional subtle nuance.

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