Where’s the proof?

Back in the 16th century, sailors in the British Royal Navy were paid a portion of their salary in rum. What they needed to test was how watered down the rum was. Some amount was already, but they didn’t want the government trying to stretch these rations at their expense.

So to test the strength of the rum a small amount of the ration would be mixed with gunpowder, and then the sailor would try to ignite it. This tested how much alcohol there was, and if the ignition was successful, the sailor had his “proof” that the rum wasn’t overly watered down.

Today, the proof of a spirit is calculated by doubling a spirit's ABV. For example, a beverage that is 10% ABV is 20 proof.