Hoof gauges are a tool used by farriers to be sure the hoof is balanced when trimmed.
A hoof gauge, divider and rule are used for determining the exact hoof angle and toe length so that a corresponding pair of feet can be trimmed the same length and angle. They are used extensively on show and racing horses, where differences in the hoof angle and toe length influence gait, way of going, and stride arc. Gauges are read from the bottom of the foot or from the side, depending on the style.