A hand is also a unit of length used to measure the height of horses. This comes from way back in the day (before tape measures were invented I guess...) when people measure the height of their horse by seeing how many of their hands it took to go from the bottom of the hoof to the top of the whithers. In modern days, a hand is equal to 4 inches. For some reason it has stuck around and even though we have a wide variety of wonderful measuring devices and excellent measurement systems, horses are almost always measured in hands...