FILE. 1. A bar of steel having sharp teeth on its surface, and used for abrading or smoothing hard surfaces. (The O. Eng. word is féol, and cognate forms appear in Dutch vijl, Ger. Feile, etc.; the ultimate source is usually taken to be an Indo-European root meaning to mark or scratch, and seen in the Lat. pingere, to paint.) Some uncivilized tribes polish their weapons with such things as rough stones, pieces of shark skin or fishes' teeth. The operation of filing is recorded in 1 Sam. xiii. 21; and, among other facts, the similarity of the name for the filing instrument among various European peoples points to an early practice of the art. A file differs from a rasp (which is chiefly used for working wood, horn and the like) in having its teeth cut with a chisel whose straight edge extends across its surface, while the teeth of the rasp are formed by solitary indentations of a pointed chisel. According to the form of their teeth, files may be single-cut or double-cut; the former have only one set of parallel ridges (either at right angles or at some other angle with the length); the latter (and more common) have a second set cut at an angle with the first. The double-cut file presents sharp angles to the filed surface, and is better suited for hard metals. Files are classed according to the fineness of their teeth (see Tool), and their shapes present almost endless varieties. Common forms are - the flat file, of parallelogram section, with uniform breadth and thickness, or tapering, or "bellied"; the four-square file, of square section, sometimes with one side "safe," or left smooth; and the so-called three-square file, having its cross section an equilateral triangle, the half-round file, a segment of a circle, the round or rat-tail file, a circle, which are generally tapered. The float file is like the flat, but single-cut. There are many others. Files vary in length from three-quarters of an inch (watchmakers') to 2 or 3 ft. and upwards (engineers'). The length is reckoned exclusively of the spike or tang which enters the handle. Most files are tapered; the blunt are nearly parallel, with larger section near the middle; a few are parallel. The rifflers of sculptors and a few other files are curvilinear in their central line.
In manufacturing files, steel blanks are forged from bars which have been sheared or rolled as nearly as possible to the sections required, and after being carefully annealed are straightened, if necessary, and then rendered clean and accurate by grinding or filing. The process of cutting them used to be largely performed by hand, but machines are now widely employed. The hand-cutter, holding in his left hand a short chisel (the edge of which is wider than the width of the file), places it on the blank with an inclination from the perpendicular of 12° or 14°, and beginning near the farther end (the blank is placed with the tang or handle end towards him) strikes it sharply with a hammer. An indentation is thus made, and the steel, slightly thrown up on the side next the tang, forms a ridge. The chisel is then transferred to the uncut surface and slid away from the operator till it encounters the ridge just made; the position of the next cut being thus determined, the chisel is again struck, and so on. The workman seeks to strike the blows as uniformly as possible, and he will make 60 or 80 cuts a minute. If the file is to be single-cut, it is now ready to be hardened, but if it is to be double-cut he proceeds to make the second series or course of cuts, which are generally somewhat finer than the first. Thus the surface is covered with teeth inclined towards the point of the file. If the file is flat and is to be cut on the other side, it is turned over, and a thin plate of pewter placed below it to protect the teeth. Triangular and other files are supported in grooves in lead. In cutting round and half-round files, a straight chisel is applied as tangent to the curve. The round face of a half-round file requires eight, ten or more courses to complete it. Numerous attempts were made, even so far back as the 18th century, to invent machinery for cutting files, but little success was attained till the latter part of the 19th century. In most of the machines the idea was to arrange a metal arm and hand to hold the chisel with a hammer to strike the blow, and so to imitate the manual process as closely as possible. The general principle on which the successful forms are constructed is that the blanks, laid on a moving table, are slowly traversed forward under a rapidly reciprocating chisel or knife.
The filing of a flat surface perfectly true is the test of a good filer; and this is no easy matter to the beginner. The piece to be operated upon is generally fixed about the level of the elbow, the operator standing, and, except in the case of small files, grasping the file with both hands, the handle with the right, the farther end with the left. The great point is to be able to move the file forward with pressure in horizontal straight lines; from the tendency of the hands to move in arcs of circles, the heel and point of the file are apt to be alternately raised. This is partially compensated by the bellied form given to many files (which also counteracts the frequent warping effect of the hardening process, by which one side of a flat file may be rendered concave and useless). In bringing back the file for the next thrust it is nearly lifted off the work. Further, much delicacy and skill are required in adapting the pressure and velocity, ascertaining if foreign matters or filings remain interposed between the file and the work, etc. Files can be cleaned with a piece of the so-called cotton-card (used in combing cotton wool) nailed to a piece of wood. In draw-filing, which is sometimes resorted to to give a neat finish, the file is drawn sideways to and fro over the work. New files are generally used for a time on brass or cast-iron, and when partially worn they are still available for filing wrought iron and steel.
2. A string or thread (through the Fr. fil and file, from Lat. filum, a thread); hence used of a device, originally a cord, wire or spike on which letters, receipts, papers, etc., may be strung for convenient reference. The term has been extended to embrace various methods for the preservation of papers in a particular order, such as expanding books, cabinets, and ingenious improvements on the simple wire file which enable any single document to be readily found and withdrawn without removing the whole series. From the devices used for filing the word is transferred to the documents filed, and thus is used of a catalogue, list, or collection of papers, etc. File is also employed to denote a row of persons or objects arranged one behind the other. In military usage a "file" is the opposite of a "rank," that is, it is composed of a (variable) number of men aligned from front to rear one behind the other, while a rank contains a number of men aligned from right to left abreast. Thus a British infantry company, in line two deep, one hundred strong, has two ranks of fifty men each, and fifty "files" of two men each. Up to about 1600 infantry companies or battalions were often sixteen deep, one front rank man and the fifteen "coverers" forming a file. The number of ranks and, therefore, of men in the file diminished first to ten (1600), then to six (1630), then to three (1700), and finally to two (about 1808 in the British army, 1888 in the German). Denser formations when employed have been formed, not by altering the order of men within the unit, but by placing several units, one closely behind the other ("doubling" and "trebling" the line of battle, as it used to be called). In the 17th century a file formed a small command under the "file leader," the whole of the front rank consisting therefore of old soldiers or non-commissioned officers. This use of the word to express a unit of command gave rise to the old-fashioned term "file firing," to imply a species of fire (equivalent to the modern "independent") in which each man in the file fired in succession after the file leader, and to-day a corporal or sergeant is still ordered to take one or more files under his charge for independent work. In the above it is to be understood that the men are facing to the front or rear. If they are turned to the right or left so that the company now stands two men broad and fifty deep, it is spoken of as being "in file." From this come such phrases as "single file" or "Indian file" (one man leading and the rest following singly behind him).  The use of verbs "to file" and "to defile," implying the passage from fighting to marching formation, is to be derived from this rather than from the resemblance of a marching column to a long flexible thread, for in the days when the word was first used the infantry company whether in battle or on the march was a solid rectangle of men, a file often containing even more men than a rank.
 This may also be understood as meaning simply "a single file," but the explanation given above is more probable, as it is essentially a marching and not a fighting formation that is expressed by the phrase.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)