A triangular file is any file that has a triangular cross section. The file has three tapered sides, which are used to file the internal surfaces of objects with acute angles. Triangular files are typically used by carpenters to sharpen the teeth of hand saws or to smooth various surfaces.
One type of triangular file that is commonly used is a knife file. A knife file resembles a flat file, but it has three sides. Two of the file's sides are wider than the other, and they taper to a point just like a knife. The file is used by applying light pressure and pushing it away then releasing the pressure and pulling it back. Files cut on the push stroke instead of the draw stroke.