The length of the shank determines the degree of leverage put on the horse's head and mouth. The leverage ratio for a typical curb bit is 1:4 (where the ratio of upper and lower cheek is 1:3), in that one ounce of pressure by the rider will result in four ounces of pressure on the mouth of the horse. Overall cheek length, from the top of the cheek ring to the bottom of the rein ring, usually cannot exceed 8½ inches for most western horse show disciplines. In Dressage the length of the lever arm portion of the cheek cannot exceed 10 cm. Cheek sizes vary from the Tom Thumb (about 4½ inches long) to bits that exceed the 8½ inch "show legal" maximum cheek length.