The tessen is an iron fan used by Japanese samurai as part of their etiquette and also to direct troops. Since in many occasions you might not have access to your swords (the daisho: a katana and a wakizashi), a tessen was frequently used as a defensive weapon. It also could be used in situations where you didn't necessarily want to kill your opponent. Some tessens didn't even fold out into the typical shape of a fan but were only used as weapons.