Bourbon is a spirit unique to America, with a history dating back to the first settlers of Scotch-Irish descend who brought their whisky-making traditions back from home. It has continually evolved and been refined over the past 200 years.
In 1964, a congressional resolution had the term "Bourbon" clearly defined. The law prescribes that Bourbon must have a minimum of two years old maturation in charred oak barrels, must be distilled under 160 proof, and be made from a mash of at least 51% corn. Bourbon which meets the above requirements may be called Straight Bourbon.
Though the law does not stipulate origin, 99% of Bourbon Whiskey comes from Kentucky. The unique limestone spring water found in Kentucky is considered by many as the only water with the ideal proportion of minerals that can yield the finest Bourbons.
The typical grain mixture for Bourbon is 70% corn with the remainder including wheat, rye and/or malted barley. After ageing, Bourbon is diluted with water and bottled. Bottling proof for whiskey must be at least 80 proof (40% abv).
Tennessee whiskey is another type of American whiskey, that bears similarity to Bourbon, in that it is composed of a mash of at least 51% corn (maize) and is aged in charred oak barrels, typically for four or more years. But unlike Bourbon, Tennessee whiskey undergoes filtering through a thick layer of maple charcoal before it is put into casks for aging. This filtering imparts the whiskey with a distinctive smooth and sweet flavour. Probably the most famous ambassador of this style is Jack Daniels Tennessee whiskey.