Hand, ancient unit of length, now standardized at 4 inches (10.16 cm) and used today primarily for measuring the height of horses from the ground to the withers (top of the shoulders). The unit was originally defined as the breadth of the palm including the thumb. A statute of King Henry VIII of England established the hand at four inches. Units of various lengths were used by the ancient Egyptians, Hebrews, Greeks, Romans, and others.