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.
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.
The butterfly sword (simplified Chinese: 蝴蝶双刀; traditional Chinese: 蝴蝶雙刀;pinyin: húdié shuāng dāo) is a short dāo, or single-edged blade, originally from the South of China, though it has seen use in the North.
Other terms
Butterfly swords are usually called 'butterfly knives' in English. However, they should not be confused with the folding Balisong (knife), which is also commonly called a butterfly knife. The Chinese word dāo is used to designate any blade whose primary function is to cut and slash regardless of length. In some branches ofKung Fu, such as Wing Chun and Wing Tsun, butterfly knives are known as Baat Jaam Do (named after the Systems form, literally 'Eight Chopping/Slashing Knives' in Cantonese).
Usage
Butterfly swords are used in several Chinese martial arts, notably Wing Chun, Shaolin, Lau Gar and Hung Gar. In Wing Chun, 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 Wing Chun practitioner will follow the same basic principles of movement as the butterfly swords. This is because the use of butterfly swords is simply an extension of empty-handed combat.
The design of the weapon, including the quillon (crossguard) shape, blade profile and blade length, are specific to each style of martial arts, the precise lineage, and individual. For example, some martial arts lineages flip the butterfly swords between the forward and reverse grip like a Sai, and consequently need a quillon that will fit the hand during a reverse grip. Some lineages trap the opponent's staff or blade between the quillon and spine, and they need a longer quillon closer and more parallel to the spine than would fit a hand after flipping. Some schools like a hybrid quillon design that is adequate for both flipping and trapping, but optimal for neither.
Chinese War Era butterfly swords had a long narrow blade that emphasized stabbing. While a deadly stabbing blade with a sharpened swage—known as "Red Boat" knives—was used by Chinese revolutionaries in the Wing Chun lineage, modern Wing Chun practitioners tend to prefer a blade profile with a wider belly that emphasizes chopping and slashing. Wing Chun lore attributes this to the desire of Monks to maim rather than kill. These knives generally have a quarter circle style tip suitable only for chopping/slashing and not stabbing, or a shallower curve to a more pointy tip that will accommodate both.
The appropriate length of the blade is a combination of the lineage and individual. For a Hung Gar stylist, the length should be a few inches past the elbow when the knife is held in a reverse grip. Wing Chun schools that use techniques which twirl the knives inside the arm need a reverse grip blade length based on the distance to the interior of the bicep. Other Wing Chun schools measure to the outside of the bicep.
Butterfly swords are regarded by many Chinese martial artists to hold the most versatility and balance of offensive and defensive capabilities of any Chinese weapon, with many more capabilities than just a weapon.