Whether wine, beer, spirit, cider or hard seltzer, every alcoholic beverage label is required to show how much alcohol it contains. What can be confusing is the multitude of ways it can be written.
The two main methods for indicating the alcoholic content of a beverage are alcohol-by-volume (abv) and proof. In the United States, a spirit’s proof is simply double the abv. This means the liquid in a bottle of 90-proof bourbon is 45% abv, while a bottle of 151-proof rum is 75.5% abv.
So, where does proof come from and why do we still use it?
Most sources point to 16th century England, where higher taxes were levied on spirits above a certain strength. Without the tools to measure the exact alcohol level of a spirit easily and accurately, its strength was tested by a much simpler method: Will it catch fire? If the liquid was strong enough to burn (or ignite a gunpowder pellet soaked in it), it was considered proof that the bottle was strong enough to warrant the extra tax.
A scale was created in which the number 100 was chosen as the “proof” at which a spirit would burn. Anything lower was exempt from the elevated tax.
This, of course, is a generally poor way to measure the amount of alcohol in a spirit. Combustibility depends on factors beyond abv, as anyone who’s ever lit a 90-proof whiskey on fire can attest. The temperature of the liquid plays an important part, too. Warmer liquid and ambient room temperate allow for more alcohol vapor to escape, increasing flammability. Absent a more scientific process, the “proof” of a liquid could change on warm day versus a cold one.
It does, however, explain why drinks like wine and beer were historically never referred to by their proof. They would never ignite and not be subject to the extra tax, so there was nothing to prove.
By the 19th century, more accurate techniques had been discovered to measure the amount of alcohol in a spirit, namely figuring out the liquid’s specific gravity, or density. As alcohol increases, spirits generally get lighter—this is how it’s possible to float different spirits on top of each other in cocktails. In England, this led to a “proof” spirit being officially defined as a one that was 12/13th the gravity of an equal volume of distilled water when measured at 11°C (51°F).
In other words, this amounted to about 57% abv being defined as a “100 proof” spirit in the United Kingdom.
If you find this math confusing, you’re not alone. Around 1848, the United States, seeking a simpler system, defined a spirit’s proof as simply double the alcohol percentage. Thus, stateside, 100 proof became synonymous with 50% abv, which is coincidentally around the same alcohol level that allows a spirit to easily ignite.
Meanwhile, a French chemist named Joseph-Louis Gay-Lussac found the arbitrary assigning of numbers by politicians setting tax standards to be inefficient, and in 1824 created his own system. For the French scale, measured in degrees, 100% alcohol was simply 100 proof, while plain water was 0 proof. In France, the proof was the abv.
Such it became that a bottle of liquor with 50% alcohol-by-volume could, at the same time, be designated as 100 proof in the U.S., 50 proof in France, and 87.6 proof in Britain.
Fast forwarding to modern times, Europe and the rest of the world has largely abandoned the convoluted proof system, and European Union guidelines came to require that the alcohol-by-volume be stated in percentage. In the U.S., the double-the-alcohol proof system is still used on some bottles, but it’s mostly a matter of tradition and only the abv is required to be listed by law.