(35 ILCS 505/1.18)
(from Ch. 120, par. 417.18)
"Proof gallon" means one gallon of 100 proof alcohol.
example: One gallon of 200 proof alcohol is two proof gallons. For the
purpose of this definition "proof" is a term used to describe the strength
of an ethyl alcohol solution and is a number that is double that of the
percent alcohol in the solution. For example: "100 proof" means 50 percent alcohol.
(Source: P.A. 82-152.)