What is 'Proof' as in spirits?

[packapoo]

Just read elsewhere that the measurement of 'proof' seems to have a different standard in UK as opposed to USA. Would that be correct? 57% UK/50% US.

(Always seemed to me that to talk proof as opposed to ABV simply set you out as a knob-jockey)

[Curmudgeon]

US proof is double UK alcohol by volume.

eg 40% abv = 80 proof, 37% abv = 74 proof.

[gaza the instructor]

Proof was set up to see if a spirit when poured
on gunpowder would still light. This is because the Navy
used to store the Gin under lock and key with the powder.
If the gin spilt on the powder it could still be lit.

[packapoo]

US proof is double UK alcohol by volume.

eg 40% abv = 80 proof, 37% abv = 74 proof.

Yes, that's what I'd always believed too so I was surprised to read that the figures differed.

Allegedly.....

[tealby]

http://io9.gizmodo.com/5871143/when-sailors-used-gunpowder-to-measure-the-strength-of-alcohol

[Curmudgeon]

https://en.wikipedia.org/wiki/Alcohol_proof