Tennessee whiskey is Straight Bourbon Whiskey produced in the state of Tennessee. This definition is legally established under the North American Free Trade Agreement (NAFTA). However, three of the four current producers of Tennessee whiskey disclaim references to their products as "Bourbon" and do not label them as such on their product bottling’s. Some Tennessee whiskies undergo a filtering stage called the Lincoln County Process, in which the whiskey is filtered through a thick layer of maple charcoal before it is put into charred new oak barrels for aging. The companies that produce whiskey in this manner say that this step improves the flavor of the whiskey. Currently, there are four brands of Tennessee whiskey on the market: Jack Daniel's, George Dickel, Collier and McKeel, and Prichard's.