The Balisong, also known as a butterfly knife, fan knife and veinte y nueve (Spanish for 29), is a folding pocket knife with two handles counter-rotating around thetang such that, when closed, the blade is concealed within grooves in the handles. It is sometimes called a Batangas knife, after the Tagalog province of Batangas in the Philippines, where it is traditionally made. In the hands of a trained user, the knife blade can be brought to bear quickly using one hand. Manipulations, called “flipping” or “fanning”, are performed for art or amusement. The knife is illegal in The Netherlands, Australia, The UK, New Zealand and Germany.
Balisongs are useful for situations where it is inconvenient to use both hands to open a knife. For example, a worker who is using a single hand to hold on to something as protection against falling will not wish to use that hand to open a knife. In such a situation it is useful to have a knife that can be opened with either hand.
The Balisong was commonly used by Filipino people, especially those in the Tagalog region, as a self defense knife and a pocket utility knife. A stereotype used to exist that every Batangueño carried one everywhere he or she went. Hollow ground balisongs were also used as straight razors before conventional razors were available in the Philippines. Due to legislation, they are no longer as common in urban areas as they were in past decades.
While the meaning of the term balisong is not entirely clear, a popular belief is that it is derived from the Tagalog words baling sungay (literally, “broken/folding horn”) as they were originally made from carved carabao and stag horn.
Balisong is also the name of a barangay in the town of Taal, Batangas province, which became famous for crafting these knives.
The traditional balisong is said to be called the veinte y nueve because they are 29 centimeters long when opened, while another story goes that it is named after a lone Batangueño who fought off 29 assailants using one.
These knives are also referred to as “fan knives” and “butterfly knives” from the motion and “click clacks” from the sound they make when they are opened and closed.
There are two main types of Balisong construction: “sandwich construction” and “channel construction”.
Sandwich constructed balisong knives are assembled in layers that are generally pinned or screwed together. They allow the pivot pins to be adjusted tighter without binding. When the knife is closed, the blade rests between the layers.
For a channel constructed Balisong, the main part of each handle is formed from one piece of material. In this handle, a groove is created (either by folding, milling, or being integrally cast) in which the blade rests when the knife is closed. This style is regarded as being stronger than sandwich construction.
Some of the blades of traditional Balisong knives in the Philippines were made from steel taken from railroad tracks thus giving them durability and hardness, while others are made from the recycled leaf springs of vehicles.
It is also used by Hit-Girl in the movie, Kick-Ass.