This one’s pretty easy. If you guessed “contains wheat,” you’ve almost got it. Bourbon’s mash bill, by law, requires at least 51% corn in its mix. If we had to make an analogy, we’d describe corn as the engine that drives the spirit. Naturally, some of the spirit’s flavor comes from this corn base, but it’s more noticeable fresh off the still, and as the whiskey ages, the corn notes fade into a general sweetness.
Next up, you have barley, which is useful for the enzymes it contains. These covert starches to sugar, which the yeast feeds on. While the barley adds a little flavor, it’s primarily a fermentation workhorse.

So that leaves us with the flavoring grains. Rye is more commonly used to add flavor to bourbons, as it brings a unique spicy note to the spirit and flavors of pepper, clove and nutmeg. You know that pleasant bite some bourbon’s have in the finish? That’s the rye. Wheat is a different story. It’s less flavorful than rye, so it allows more of the corn’s sweetness and vanilla from the barrel to come through. Some might see it as a slightly flavor-neutral grain, but its defenders believe that wheat retains its flavor over the course of extreme aging (10 to 20 years). So a wheated bourbon still contains at least 51% corn (typically 70 to 80%), but the remainder of the balance will be divided between barley and wheat.