Until the Middle Ages, spirits were matured in clay vats, but then oak barrels came into vogue, making it easier to transport the product. Oak also has an impact on the flavor of the distillate, adding flavor and sometimes color slowly during the aging process.
By the 17th century, barrel aging was widely used in Cognac and Armagnac, two regions famous for their brandies in southwest France. This trend spread to Scotland in the 18th century and then to North America in the 19th century. Gradually, oak came to dominate the industry, but even today some other types of wood are used, like the jequitiba wood that's used to age some cachaça. The importance of wood cannot be overstated and some believe that the impact accounts for up to 70% of the total flavor. Specific flavors imparted vary wildly depending on the type of oak (French, American, new, old, etc.) used. Cask maturation allows some alcohol to escape, reducing the alcohol gradually and softening the spirit. This evaporation is known as "the angel's share."
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