Proof originally referred to "gunpowder proof." One of the ways early distillers checked, or "proved," that their whiskey was strong enough was by mixing equal parts of whiskey and gunpowder, then setting the mixture on fire. A slow, yellow flame meant the spirit was too strong, while a steady blue flame "proved" it was good whiskey. Yellow flame spirits were dilluted with water until they burned blue. Later, the whiskey which burned blue was found to be 50 percent alcohol. This level was considered to be 100 percent proved or, as we know it today, 100 proof.

Today, the alcohol content is measure more scientifically in the distillery using a hydrometer. Proof is still calculated as 2 times the "alcohol by volume" of a given Bourbon.