The butterfly sword is a short dao, or single-edged sword, originally from southern China, though it has also seen use in the north. It is thought that butterfly swords date from the early 19th century. Several English language accounts from the 1840s describe local militia in Guangdong being trained in the "double swords", short swords with a hook extending from the guard, and fitting into a single scabbard. The blade of a butterfly sword is roughly as long as a human forearm, which allows easy concealment inside loose sleeves or boots, and allows greater maneuverability when spinning and rotating during close-quarters fighting. Butterfly swords are usually wielded in pairs. A pair of swords will often be carried side by side within the same scabbard, so as to give the appearance of a single weapon. The butterfly sword has a small crossguard to protect the hands of the wielder, similar to that of a sai, which can also be used to block or hook an opponent's weapon. In some versions the crossguard is enlarged offering a second handhold, held in this position the swords can be manipulated in a manner akin to a pair of tonfa. They may also be used as brass knuckles when non-lethal application of the weapon is desired. Traditionally, the blade of a butterfly sword is only sharpened along half of its edge – from the middle of the blade to the tip; this can be seen in all vintage specimens from the Qing dynasty. The blade from the midpoint down is left blunt so that it can be used to deliver non-lethal strikes and to block without damaging the sharpened edge. Butterflies were generally commissioned for individual martial artists, not mass-produced, so every set of swords is different; however, an average blade today is about " long with a 6" handle. Other terms Butterfly swords are usually called 'butterfly knives' in English. However, they should not be confused with the folding balisong, which is also commonly called a butterfly knife. The Chinese word dao is used to designate any blade whose primary function is to cut and slash regardless of length. In some branches of Kung Fu, such as Wing Chun, butterfly knives are known as Baat Jaam Do (named after the system's form, literally 'Eight Chopping/Slashing Knives' in Cantonese). Use Butterfly swords are used in several Chinese martial arts, notably Wing Chun, Hung Ga, and