With distilled spirits (such as bourbon, scotch, vodka, gin, and rum), alcohol content is expressed in terms of proof. For example, 100-proof gin is 50 percent alcohol.
'Proof' is directly two times the alcohol by volume or ABV (measured as a percentage).
The term 'proof' originated from England, where the government used to impose a greater tax on liquor having a greater quantity of alcohol. Alcohol content was decided by means of a quite primitive test.
Primarily, the government would rinse a gun pellet with alcohol and seek to ignite the gunpowder. If it was ignited, the alcohol content in the liquor was considered high enough to be judged to be a 'proof' spirit, in the sense that there was proof which indicated that it had a greater quantity of alcohol.
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