Originally meaning "of tried strength or quality", this acquired new meaning with the invention of the hydrometer - a floating instrument used to determine the specific gravity of a fluid - in this case an alcohol/water mixture. The definitions were progressively firmed up via published tables but for purposes of argument, British "proof spirit" contains 57.1% alcohol by volume or 49.28% alcohol by weight at 51 deg F. American proof spirit by contrast contains 50% alcohol by volume at 60 deg F. 100 deg proof British spirit therefore corresponds to 114.2 def proof in the USA, similarly American 100 deg proof spirit is 87.7 deg proof British. On this scale incidentally pure alcohol rates 175 deg proof (British).