The butterfly sword (Traditional Chinese: húdié shuāng dāo) also known as dit bang dao (life-taking sword) or yee jee dao (character two sword), is originally from the South of China, though it has seen use in the North. The blade length is approximately that of the forearm, for easy concealment within the sleeves or inside boots, and for greater maneuverability to spin and rotate in close-quarters fighting. The butterfly sword is usually wielded in pairs. As well, they are usually held side by side within the same scabbard, so as to give the appearance of a single weapon. The sword has also a small crossguard to protect the hands of the wielder, which can be used to block or hook an opponent's weapon. The guard can also be used as a knuckle duster when non-lethal application of the weapon is desired. Traditionally, the blade of a butterfly sword is only sharpened halfway, from the middle of the blade to the tip. From the middle down to the handle, the blade is left blunt. This is done so that the unsharpened portion of the blade can be used for blocking without damaging a finely honed edge, and to deliver non-lethal strikes. Butterfly swords are used in several Chinese martial arts, notably WingTsun( known as Baat Cham Dao), Lau Gar and Hung Gar.
 

	Wing Tsun Bart-Cham-Dao  (Wing Tsun Doubleknives)

In WingTsun, one notable aspect of butterfly sword combat is that its principles are the basis for all other weaponry. In theory, any object that can be held in the hands of a WingTsun practitioner will follow basically the same principles of movement as the butterfly swords. This is because the use of butterfly swords is simply an extension of empty-handed combat.

Butterfly swords are regarded by many Chinese martial artists to hold the most versatility and balance of offensive capabilities and defensive capabilities of any other Chinese weapon. with many more capabilities than just a weapon.

Sources: http://en.wikipedia.org/wiki/Butterfly_Sword,

www.kultofathena.com