First, and most importantly, remember: all bourbon is whiskey, but not all whiskey is bourbon. The main difference between the two is in the fermentation process - the grain used and how it is stored/aged. By definition, whiskey is a spirit distilled from fermented grain, including wheat, rye, barley, and corn—and then aged in wooden barrels. To legally call a whiskey a bourbon, it must be: - produced in the United States. (Other whiskeys are produced all over the world; most common are Scottish and Irish.) - made from 51 percent corn. (For other whiskeys, the mix of grains can vary.) - distilled to a max of 160 proof and placed into the barrels at a max of 125 proof. (Other whiskeys can be distilled to a max of 190 proof.) - stored in newly charred white oak barrels. (For other whiskeys, then need to be in oak barrels, but can be new or old, charred or not.) In terms of flavor, bourbon tends to be slightly sweeter and smoother than other whiskeys. Any whiskey - bourbon or other varieties - works well in this cocktail.