Alcohol proof

The term was originally used in England and from 1816 was equal to about 1.75 times the percentage of alcohol by volume (ABV).

In 1972, Canada phased out the use of "proof"; in 1973, the European Union followed suit; and the United Kingdom, where the concept originated, started using ABV instead in 1980.

The term proof dates back to 16th century England, when spirits were taxed at different rates depending on their alcohol content.

This test relies on the fact that potassium nitrate (a chemical in gunpowder) is significantly more soluble in water than in alcohol.

Factors including the grain size of gunpowder and the time it sat in the spirit impact the dissolution of potassium nitrate and therefore what would be defined as 100 proof.

By the end of the 17th century, England had introduced tests based on specific gravity for defining proof.

The proof system in the United States was established around 1848 and was based on percent alcohol rather than specific gravity.

Today, liquor is sold in most locations with labels that state its percentage alcohol by volume.

On 1 January 1980, Britain adopted the ABV system of measurement prescribed by the European Union, of which it was then a member.

The Code of Federal Regulations[9] requires that liquor labels state the percentage of ABV at a temperature of 60 °F (16 °C).

The regulation permits, but does not require, a statement of the proof, provided that it is printed close to the ABV number.