Proof is an anglo-american unit for the alcohol content of alcoholic beverages. The system dates to the 18. Century, when gun powder was moistened with the alcohol solution. If this was still explosive, the solution was "proofed". Later a "proven" solution was defined as 100 degrees proof (100o). This corresponds to an alcohol content of 57,15 percent by volume (ABV, Vol%).
The proof scale of Great Britain and the USA are different: In Great Britain approximately a relationship of 7:4 can be set for the conversion by Proof in Vol% (70o Proof = 40 Vol%). Pure alcohol is therefore 175o Proof. The precise measurement of the proof content takes place by means of a hydrometer.
In the USA the "proof number" (without degree character) is defined as the double of the alcohol content in percent by volume at a temperature of 60o F (15,5o C). Therefore, a content of 80 proof correspond to 40 Vol% and pure alcohol is 200 proof. According US Federal regulation (CFR 27 5.37 Alcohol Content) the alcohol content is indicated nowadays in percent by volume, the indication of proof may take place additionally.