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How is alcohol proof measured in distilled liquor?

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distilled spirit The fermentation and distillation process for producing whiskey. The production of whiskey begins with grinding grain into a meal, which is cooked. Malt is introduced to the meal, which results in mash that is cooled and pumped into a fermenter, where yeast is added. The fermented mixture is heated in a still, where the heat vaporizes the alcohol. The alcohol vapours are caught, cooled, condensed, and drawn off as clean, new whiskey. This liquid is stored in a cistern room, and water is added to lower the proof (absolute alcohol content) before the whiskey is placed in new charred oak barrels for aging and later bottling.

In distilled liquor, alcohol proof is a measure of the absolute alcohol content, typically determined by calculating the specific gravity of the liquid—the weight per unit volume compared to water at a specific temperature. Because specific gravity changes with temperature, measurements must be taken at a standardized index temperature.1

The definition of proof varies by region:

United States: Proof is defined as two times the alcohol by volume (ABV) percentage. For example, a spirit that is 50% alcohol by volume is 100 proof.234
Great Britain: Historically based on a 16th-century flammability test, the modern standard (codified in 1952) defines 100 proof as a spirit with a specific gravity 12/13 that of an equal volume of distilled water at 51°F (11°C), which corresponds to approximately 57.06% ABV.12
France (Gay-Lussac): This system uses a direct scale where 100% ABV equals 100 proof and 0% ABV equals 0 proof, making the proof number identical to the ABV percentage.23