measuring tool called the pied du roi ("foot of the king"), invented between the 1500s to the late 1700s. 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 to 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. Construction 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. One notable example would be the Tsunami from Squid Industries. 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