A butterfly knife is a type of folding pocketknife that comes from the Philippines. It is also called a Balisong, a fan knife, or a Batangas knife. It has two handles that rotate in opposite directions around the tang. When the knife is closed, the blade is hidden in grooves in the handles. The handles are held together by a latch, which is usually placed on the handle facing the cutting edge (the "bite handle").
Filipinos, especially those from the Tagalog area, used the balisong as a self-defense tool and a pocket knife. Before regular razors were available in the Philippines, hollow-grind balisongs were also used as straight razors. The knife blade can be used quickly with one hand by someone who knows how to use it. "Flipping" is the name for manipulations that are done for art or fun. There are blunt "trainer" versions of these knives that can be used to practice tricks without hurting yourself.
Some countries have made it illegal or limited to carry the knife, often for the same reasons that switchblades or concealed weapons are not allowed. In the cities of the Philippines, it is not as common as it used to be. The history of the butterfly knife, or "Balisong" as it's known in the Philippines, where it's thought to have come from, is not very clear.
Many people who practice Filipino Martial Arts say that the Balisong came from a small town called Balisong in the province of Batangas, which is just south of the country's capital, Manila. People say that a man from Balisong town named Perfecto de Leon "invented" the famous blade in his small shop in the year 1905. At the time, the Philippines was a US colony. It's likely that some members of the colonial army "discovered" the locally made blade and brought it back to the US, along with a story that a Filipino was the "original inventor" of the butterfly knife.
The story that the Balisong was made in the Philippines is not true, because there have been many knives made in Europe that look like the ones made in Balisong town. These blades can be traced back to a German company called Bontgen and Sabin, which made pocket knives, camping knives, spring knives, and even bayonets in World War I. They were made in 1880. Their "folding knife" design is very similar to the Filipino Balisong. It was even patented in Germany and the United States.
We may never really know how Bontgen & Sabin's folding knife became the "Balisong" of the Philippines, but it is thought that Perfecto de Leon saw one of these knives when the Philippines were still a Spanish colony, copied the blade, and started making them in large numbers. The Balisong was popular among US servicemen from 1905 until 1992, when both US military bases in the Philippines (Subic Naval Base and Clark Air Base) were closed. The Balisong was of high quality and inexpensive compared to other blades. The Balisong is still a popular export to the US, and many American blade makers make them in different styles using space-age materials and advanced manufacturing techniques.