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The measure of alcohol strength in a spirit.
Proof spirit (or 100% proof spirit) acquired its name from a solution of alcohol in water which, when used to moisten gunpowder, contained just enough alcohol to permit it to burn. Anything above or below this strength is termed under (U.P) or over (O.P) proof.
In a alcoholic drink, the degrees proof is based more accurately on the specific gravity of the solution, and this depends on temperature.
Measurements of this are confusing, and differ slightly between Britain and the U.S.A.
In the U.S.A "proof" of deemed to be 50% alcohol by volume at 60ºF (16ºC). In Britain, "proof" is 49.28% of pure alcohol by weight or 52.10% by volume, at 60ºF (16ºC).
Each 0.5% degree of alcohol over or under "proof" is reckoned as one degree O.P or U.P. Alcoholic content in a spirit is usually stated as so many degrees proof - 70º
proof (or correctly, of proof, though this is seldom, if ever, stated) means the same as 30º under proof, in either case seventy parts of proof being present and thirty lacking.
100% pure alcohol, measured thus is in fact 75.35 O.P.