Proof is a measurement of the alcohol content of bourbon or other whiskeys. Typically it is twice the ABV (alcohol by volume or the percentage of alcohol in the liquid). So, if a bourbon contains 50% ethanol, the proof would be 100. However, the proof scale can vary by country and historical reference. For instance, before 1980, the U.K. used a ratio of 4 to 7 to determine the proof. At that time, you would need to multiple the ABV by 1.75 (not 2) to get the proof. 

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