How did this start, and why? You have to know the percentage anyway to measure proof, what is gained by doubling it and calling it “proof”?
Why not just use percentage?
WAG: Marketing gimmick: higher number makes it sound more potent.
Nope, historical reasons.
Originally, distilled liquors were “proved” by soaking gunpowder with them and lighting the mixture. If it went out, the liquor was “under proof strength”; if it flared, the liquor was “over proof strength”. If it burned steadily, it was “at proof”. (This is the strength at which it was intended to be distributed, not the higher (often much higher) strength at which it was distilled).
Now, “proofed” liquor is about 43.4% alcohol. At some time in the past, someone divided the other 56.6% into one hundred degrees (a nice round number), so that one could refer to a stronger (or weaker) liquor than usual as “so-and-so many degrees over (or under) proof”. This terminology is seen in some writings as late as 1900.
Incidentially, this (British) “degree of proof” is not 1/2 of 1%, it is about 4/7 of 1%. This makes proof spirit almost exactly 76°; whether this was a consideration in adopting the size of a degree of proof, I don’t know.
In the U.S., it was decided to ignore all of this historical jazz and just call a degree of proof 1/2 of 1%. In recent years, of course, the standard proof sold has gradually declined from 86.8°, to 86°, to 80°. That’s probably sheer cheapness on the part of bottlers.
Fermented beverages have never, SFAIK, had their strengths measured in degrees of proof, but in percent alcohol by volume (Canada) or weight (U.S.), or by the original gravity of the wort or must (U.K.)
FTR, I drink my bourbon considerably over proof. Some deprecate this habit, feeling that the burn of alcohol conceals the other flavors in high-proof liquor; in fact, proof spirit may have as much as 50% water added to it before bottling
It is often said that “anything is possible”. In fact, very few things are possible, and most of them have already happened.
According to my Webster’s - it’s a handy thing, you should get one - alcoholic strength is measured against a standard concentrantion that is called a proof spirit which is 50% alcohol by volumne (in G.B. and Canada 57.1%) at 60ºF.
My Jesus fish can beat up your Darwin fish but forgives it instead.
Thanks Ak!
Padeye, the dictionary definition only explains the relationship of percentage to proof, which I knew. It doesn’t explain why they simply don’t use percentage, which was my question.
And I have several dictionaries, thanks.
Easy, Easy… put down that massive tome, someone could get a paper cut ! Now, hug and make up. 
Cartooniverse
If you want to kiss the sky, you’d better learn how to kneel.