What is the Imperial Gallon ?

Source: en.wikipedia.org

" The British imperial gallon is defined as exactly 4.54609 dm3. It is used in some Commonwealth countries, and until 1976 was defined as the volume of 10 pounds (4.5359237 kg) of water at 62 degrees Fahrenheit (16.67 °C).There are four quarts in a gallon, the imperial pint is defined as 0.56826125 litres (1⁄8 gallon) and there are 20 imperial fluid ounces in an imperial pint, yielding 160 fluid ounces in an imperial gallon. "