The proof system in the United States was established around 1848 and was based on percent alcohol rather than specific gravity. 50% alcohol by volume was defined as 100 proof. Note that this is different from 50% volume fraction (expressed as a percentage); the latter does not take into account change in volume on mixing, whereas the former does. To make 50% ABV from pure alcohol, one would take 50 parts of alcohol and dilute to 100 parts of solution with water, all the while mixing the solution. To make 50% alcohol by volume fraction, one would take 50 parts alcohol and 50 parts water, measured separately, and then mix them together. The resulting volume will not be 100 parts, but between 96 and 97 parts, since the smaller water molecules can take up some of the space between the larger alcohol molecules (cf. Volume change).