The relation of the purchase arm—the length from the mouthpiece to the cheekpiece rings—and the "shank" or lever arm—the length from the mouthpiece to the lowest rein ring, is important in the severity of the bit. A long lower shank in relation to the upper shank increases the leverage, and thus the pressure, on the curb groove and the bars of the mouth. A long upper shank in relation to the lower shank increases the pressure on the poll, but does not apply as much pressure on the bars of the mouth.