The term “proof spirits” was first used as a yardstick in collecting taxes on alcoholic beverages. Originally, the British tested spirits by pouring them on a small amount of gunpowder. If the gunpowder burned, the spirit was called “over proof”. If it didn’t burn, the spirit was “under proof”. Then scientists found new methods of analysis. They learned that “proof spirit” was 57% alcohol and 43% water by volume.
The Americans now use a different system. Their “proof spirit” is 50% alcohol and 50% water. To find the strength of American-bottled alcohol, just divide the proof strength by two. For example: 80% U.S. Proof means 40% alcohol.
In Canada, as in England, liquors were labelled “30 under proof”. This meant 30% less than the British proof strength (57% alcohol). Thus, Canadian “30 under proof” was 70% of 57%, or approximately 40.46% alcohol, which is the same as the American “60 proof”. Don’t wreck your brain over it. At the end of the day, it was just meant to be an interesting fact!