Shuriken (手裏剣; lit: "swor hidden in the hand") is a traditional Japanese concealed weapon that were generally used by shinobi no mono or ninja, as they are mainly known as, for throwing, and sometimes stabbing or slashing. They are sharpened hand-held blades made from a variety of everyday items such as needles, nails, and knives, as well as coins, washers, and other flat plates of metal. Shuriken were mainly a supplemental weapon to the more commonly used katana (sword) or yari (spear) in a warrior's arsenal, though they often played a pivotal tactical role in battle. The art of wielding the shuriken is known as shuriken-jutsu, and was mainly taught as a minor part of the martial arts curriculum of many famous schools, such as Yagyu Ryu, Katori Shinto Ryu, Itto Ryu, Kukishin Ryu, and Togakure Ryu. In the modern western world, shuriken can often be purchased online as collector's items, but in some countries owners must possess a certificate for "knives".