Alcohol by Volume (or ABV) is a percentage. It’s simple and easy to understand. We learned about percentages in grade school. But what is proof, and why are we still using it? The short answer is tradition. The long answer is a little more interesting.

In the United States, proof is defined as twice (or double) the percentage of ABV. A proof statement is legally permitted to be on a bottle, provided it’s near the ABV, but it’s not required. So why even have it? Because the spirits industry loves tradition, and this one goes back to 16th century England.

Being a British sailor back then was a hard life. They hadn’t really figured out the whole scurvy/citrus connection yet, you often worked barefoot in frigid seas, and had exotic new diseases waiting for you in every port. Oh, and pirates were an actual career risk. So we would hope that these men were handsomely compensated for their labor. Turns out, not really.

Rum was often a common form of wages paid to sailors—specifically, strong rum. And these sailors wanted their just dues. In order to avoid accepting watered-down rum from unscrupulous captains, they figured out a pretty ingenious test. Alcohol at a percentage under 57.15% ABV has a significant tell: if you douse gunpowder with it, the powder won’t light. If you try it with rum at 57.15% or over, the powder sparks just fine, thus proving the spirit’s “proof.” It should be said: we neither condone nor recommend mixing flammable alcohol and gunpowder on a wooden boat. However, that’s how alcohol percentage was commonly measured for a couple of hundred years. In the 18th century, the gunpowder test was replaced by the more accurate specific gravity test, but the name and the legend have endured.