4 Inches Wide at the Knuckles...

The practice of determining horse height in 'hands' stems from the notion that the average man's hand is approximately 4 inches wide across the knuckles (one hand = 4 inches).  Horses are measured from the ground to the highest point on their shoulders; the 'withers'. Anything under 14.2 hands (hh) is considered a pony, and anything between 14.2 - 16.2 hands is a 'light' horse.