system
Given the following post, respond as if you're commenting on Reddit:
human
Why is alcohol measured in proofs?

I understand that a proof is double the percentage of alcohol by volume, but if that measurement is a universal rule, why not just use percentages; why use proofs at all?
gpt
From the 18th century until 1 January 1980, the United Kingdom measured alcohol content in terms of "proof spirit", which was defined as spirit with a gravity of 12/13 that of water, or 923kg/m3, and equivalent to 57.15% ABV.[1] The term originated in the 18th century, when payments to British sailors included rations of rum. To ensure that the rum had not been watered down, it was "proved" by dousing gunpowder with it and then testing to see if the gunpowder would ignite. If it did not, then the rum contained too much water and was considered to be "under proof". Gunpowder would not burn in rum that contained less than approximately 57.15% ABV. Therefore, rum that contained this percentage of alcohol was defined to have "100° (one hundred degrees) proof".