A type of hammer with an iron head and an oak handle. The gennō is used to strike a chisel or plane, or to drive nails. The iron head has two striking faces; one is flat, and the other bulges out slightly in the center. This convex face is called kigoroshimen 木殺面 and is used, for example, to lightly tap the handle of a chisel in the last stage of driving a nail. Gennō are classified by the weight of the head: those with 650-750g heads are called, daigennō 大玄能; with 400-500g heads, chūgennō 中玄能; and with 250g heads, shōgennō 小玄能. The length of the gennō handle varies according to the weight of the head, but a length of about 300 mm is standard. The profile of the handle is rectangular at the blade entrance, (the expression gohira 五平 is sometimes used), but oval where it is held in the hand, giving an easy grip.