The "bō" is usually made with unfinished (no varnish, stain, etc) hard wood or a flexible wood, such as red or white oak, although bamboo and pine wood have been used; more common still is rattan wood for its flexibility. The modern "bō" may be tapered in that it can be thicker in the center ("chukon-bu") than at the ends ("kontei") and is usually round or circular ("maru-bo"). Some bō are very light, with metallic sides, stripes and a grip which are used for XMA and competitions/demonstrations. Older bō were round ("maru-bo"), square ("kaku-bo") ("rokkaku-bo") or octagonal ("hakkaku-bo"). The average size of a bō is 6 shaku (around ) but they can be as long as ("kyu-shaku-bō").