Imperial (used in the British Empire) vs US customary. The imperial fluid gallon (4.54609 L exactly) was never historically defined in terms of another unit while the US fluid gallon was defined as 231 cubic inches (3.785411784 L exactly). A pint is defined as 1/16 of a gallon in each system, but they can't agree on how many ounces are in a pint (16 for US, 20 for imperial). Note that there are also imperial and US customary dry gallons and thus imperial and US customary dry pints...