The relative ratio between the length of the purchase and the lever arm also affects the amount and type of leverage that is applied to the chin and poll of the horse (producing 1:3 ratio of rein to chin+poll forces in case of the typical curb bit). A long lower shank (lever arm) in relation to the upper shank (purchase) increases the leverage, and thus the pressure, on the curb groove and the bars of the mouth. This design is best suited for a longer-necked horse, as it encourages the horse to both drop its head down and bring its nose in. A somewhat 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. This design is often more helpful on a horse with a short and thick neck, as it encourages the horse to drop its head, but with less pressure to flex the nose in, an act that is physically more difficult for a horse with a thick neck.