The imperial system is yet a third system, formerly used in the British Empire and a few other countries. There are important differences between it and the U.S. customary system.

For instance, the imperial gallon is about 4.5 liters, while the U.S. gallon is only about 3.78 liters.

The imperial system used to be much worse... there were separate gallons for wine, ale, grain, etc. About the only vestige of this is in barrels, where a barrel of beer is not the same size as a barrel of oil.

There's also an avoirdupois system, which is similar, but once again, not identical, with the imperial and U.S. systems, and the troy system used for precious metals and things of that nature.