Why Tennessee Whiskey Isn't Bourbon

What is sold as Tennessee whiskey is not legally bourbon, despite being made from bourbon ingredients, because of the Lincoln County Process, a step in which the distillate is filtered through 10 feet of sugar maple charcoal before being barreled for aging. The process, used by both Jack Daniel's and George Dickel, was ruled by Tennessee law to make the spirits a distinct product from bourbon. Jack Daniel's, founded in Lynchburg, Tennessee in 1866, is the best-selling American whiskey in the world and is made in a county that is officially dry, meaning you cannot legally buy a drink at a bar in the town where it is produced.

Source: Wikipedia