Butterfly knife

A butterfly knife, also known as a balisong, fan knife or Batangas knife, is a type of folding pocketknife that originated in the Philippines. Its distinct features are two handles counter-rotating around the tang such that, when closed, the blade is concealed within grooves in the handles. A latch sometimes holds the handles together; typically mounted on the one facing the cutting edge (the "bite handle"). An exceptionally large balisong is called a balisword.
The balisong was commonly used by Filipinos, especially those in the Tagalog region, as self-defense and a pocket utility knife. Hollow-grind balisongs were also used as straight razors before conventional razors were made available in the Philippines. In the hands of a trained user, the knife blade can be brought out to bear quickly using one hand. Manipulations, called "flipping", are performed for art or amusement. Blunt "trainer" versions of these knives are also available and can be used to practice tricks without the risk of injury.
The knife is now illegal or restricted in some countries, often under the same laws and for the same reasons that switchblades or concealed weapons are restricted. Within the Philippines, it is no longer as common in urban areas as in the past.
Names for the knives in English include "fan knives" and "butterfly knives" from the motion, and "click clacks" from the sound they make when they are opened and closed.
The name "balisong" is derived from barangay (village) Balisong, part of the municipality of Taal, Batangas province, which along with the neighboring barangay Pandayan (now part of Poblacion zones 6, 9, and 10 of Taal town proper), were the original manufacturing centers of the knives in the Philippines. The two barangays were home to a blacksmith industry that also produced other bladed implements such as bolo knives. It is also claimed that the meaning of the term balisong is derived from the Tagalog words baling sungay (literally, "broken/folding horn") as the hilt of the blade was traditionally made from carved carabao and deer horn, as well as bones. The traditional balisong is also known as veinte y nueve or "twenty-nine" in the Philippines because they are 29 cm (11 in) long when opened.
The origin of the knives is unclear. Oral histories claim that the knives were first created in the Philippines in 800 CE. However, there is no documentation or archeological evidence to back this. Balisong's mass production in the Philippines can only be attested to the early 1900s. Another claim is that balisong was originally an adaptation of a French measuring tool called the pied du roi ("foot of the king"), invented between the 1500s to the late 1760s. However, how it was introduced to the Philippines is unknown. There are theories that it may have been introduced by sailors in the Spanish Empire, which was then allied with France.
Regardless of origin, the modern balisong was perfected in the Philippines, where it became much larger, and was predominantly used as a weapon, not just a tool. The quick opening techniques ("flipping") were also developed in the Philippines. In contrast, the French pied du roi was primarily a folding ruler, with the knife only included in some specimens as a novelty. They were cumbersome to open and unlikely to be used for self-defense, especially since they also commonly included a metal tang at a right angle from the end of the handle to aid in measuring. There were also very similar designs to the balisong produced in England in the late 19th century, presumably also derived from the pied du roi. But like the latter, they were primarily utilitarian tools.
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 more tightly 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.
Additionally, the two constructions can be combined to form the "chanwich construction", which involves two halves of a channel handle screwed together. Although rare, this construction generally keeps the best elements of both constructions and discards the worst, as it retains the better handle shape channel construction is known for, while still allowing adjustment of the tightness the handles are held together with to some extent, as well as easier access to the inside of the handle for cleaning.
There are also three methods of operation balisongs use: bearings, bushings, or only washers.
Bearing-operated balisongs have small ball bearings housed in a circular concavity around the hole in the pivot. These bearings allow the handles of the balisong to rotate.
Bushing-operated balisongs have a small metal bushing slightly thicker than the tang in each pivot hole with a usually bronze disc known as a washer on each side. These washers clamp down on the bushing, but not on the tang, when the pivot screw is tightened, allowing the handle to rotate around the tang.
There are also washer-only operated balisongs which are usually much cheaper and lower quality than the other kinds, as they don't need bushings, but the handles will always bind to the tang when the screws are tightened enough and the washers, tang and handles all wear themselves down much faster due to the increased friction.
Some of the blades of traditional butterfly knives in the Philippines were made from steel taken from railroad tracks, thus giving them a decent amount of durability and toughness, while others are made from recycled leaf springs of vehicles.
Most modern balisongs do not use tang pins. Instead, they use "zen pins", which are two small pins embedded in the top of the handles of the balisong which make contact with the bottom of the blade. While a balisong with zen pins is easier for end user to perform maintenance on for the end user and significantly easier to design; they do not hold up as long as tang pins would in a properly made knife. the force of the harder blade contacting the soft zen pins will mean they need replaced far more often than tang pins which should be mild steel so the blade doesn't crack and can cushion against soft handle materials rather than crush each other.
Some modern balisongs also have a "pinless" system, in which they do not have any pins, and instead rely on the material of the handles to make contact with the blade, similar to how zen pins work. The longevity of pinless is exceedingly short and not meant for long term usage before the system itself fails and the knife breaks.
The balisong has been outlawed in several countries, mainly due to its easy utility in crimes and its ability to be easily concealed for the same purpose. In some jurisdictions, its criminal use is considered a knife crime.
Balisong trainers feature a special blunt and unsharpened "blade" and are legal in some areas where balisongs are not.