Butterfly knife

A butterfly knife, also known as a fan knife and in the Philippines as the balisong, is a folding pocket knife. Its distinction is two handles counter-rotating around the tang 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.

The balisong was commonly used by Filipino people, especially those in the Tagalog region, as a self-defense and pocket utility knife. A common stereotype is that a Batangueño carries one everywhere he or she goes. Hollow-ground balisongs were also used as straight razors before conventional razors were available in the Philippines. 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. Blunt versions of these knives, called "trainers", are for sale to practice tricks without the risk of injury.

The knife is now illegal or restricted in many countries, often under the same laws and for the same reasons that switchblades are restricted, and in their country of origin they are no longer as common in urban areas as they were.

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belief is that it is derived from the Tagalog words baling sungay (literally, "broken/folding horn") as they were originally made from carved carabao and stag horn.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 though may sometimes use a ball-bearing system. They allow the pivot pins to be adjusted tighter without binding. When the knife is closed, the blade rests between the layers.

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Wikipedia