Why are most liquors 80 proof?

[Gagundathar]

@Kinthalis

A related question:

Why proof? Who came up with this measurement? It looks like proof = twice the percent of alcohol by volume right? Why not just use alcohol by volume?

I have often wondered the same myself.

According to this relatively reliable source (Wikipedia):

From the 18th century until 1 January 1980, the United Kingdom measured alcohol content in terms of “proof spirit”, which was defined as the most dilute spirit that would sustain combustion of gunpowder.[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 in it, then tested 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”.

Drats! Did not read far enough in the dang thread!
Sorry.
Kenobi is right as far as I can tell.