Most of us casually use the word proof without actually knowing or understanding what it means. Proof is the decided upon scale used to measure the amount of alcohol in the liquor. Legally speaking, a modern proof signifies twice the percentage of alcohol by volume. The scale goes from 1 to 200, with a 200 proof (100% alcohol) signifying an absolute alcohol, and a 100 proof (50% alcohol) signifying a proof spirit.
Now the above definition applies to the United States proof system. Although very similar there are minute differences in the definition based on where you are. For example, in the United Kingdom the alcohol proof system is a 4:7 alcohol by volume ratio in comparison to the U.S. 1:2 ratio. This means you have to multiply the alcohol content by 1.75 to give you the proof.
Overall, the proof of an alcohol isn’t too hard to figure out since its always printed with the alcohol content right next to it on every bottle. In other words, it’s completely redundant and kind of useless; so why is it there?
Well we can bring up the British again if we’re talking about where the origin of proof comes from. You have to go back about 300 years to a time before we can use instruments of science to determine the density of liquids. There is a strong belief that the British paid their soldiers in money and rum back then, and to prove the rum was not watered down they would pour the rum on gunpowder and try to light it. If it failed to ignite, that would mean the rum had too much water and was under proof. If the gunpowder caught fire and eventually sparked, then that was proof. The rum had to be at least 57% alcohol and 43% water to ignite, so that in the above mentioned system; a British 100 proof rum was 57% alcohol by volume. Now of course it could be more than 57% alcohol which resulted in a rum being over proofed, something that still exists today. (Looking at you Everclear)
Following the trends of many things in history, the name “proof” stuck. The hydrometer would be invented in the 1700s and would end the crude gunpowder proof test. However, consumers trusted the proof name and had the expectation of it on their bottles so the name continued.
The U.S. adopted their own measurement of alcohol by volume (ABV) in the mid 1800s and now all modern distillers are required to print the ABV on the bottle. They are also allowed to print the proof usually right next to it, which again is redundant.
Today, the proof of an alcohol is largely used for simple marketing purposes instead of being an actual measurement. Bacardi 151® sounds much cooler than Bacardi® 75.5ABV right?