The US customary fluid ounce is based on the Queen Anne wine gallon of 231 cu in. The Imperial weights and measures reform in the 1820s defined a new gallon equal to the volume of 10 lb avoirdupois of water at s.t.p., which was about the same as the old ale gallon. The Imperial pint was changed to be 20 fl. oz. instead of 16 fl. oz. so that the metric-style correspondence between mass and volume also worked for ounces.