Prior to World War II, the butterfly sword was not well-known outside of China. The deadly swords feature a single-edged blade that is as long as a human forearm. This length allowed for concealment inside loose sleeves or boots. Typically, butterfly swords are wielded in pairs.
Butterfly swords, also known as butterfly knives, were only sharpened from the midpoint to the tip. This way, the dull part of the battle knives could be used to knock an opponent unconscious without being fatal. Another interesting fact is that butterfly swords were often kept in the same scabbard. This gave the illusion of a single weapon until the wielder was ready to attack.