Two straight-edges which are of similar dimensions are named a pair, and if they are also right-angled parallelopipeds, and are thick enough to stand on their small long sides as bases, the two tools are twist-finders. Two such tools are used to ascertain if one surface is inclined towards another, or inclined to some other part of the same surface to which the twist-finders are put. This angular inclination is termed the twist, and to discover it the two finders are stood with their edges in contact with the surfaces that are supposed to have a twisted relation to each other; and while the finders remain in position, an observer stands a few feet from the work and looks across the upper edges of the two tools. If the surfaces that are being tried are either parallel to each other or in the same plane, the upper edges of both tools will appear to be parallel to each other; but if one surface is inclined to the other it is detected by one end of one straight-edge being seen to be too high, and by the opposite end of the same straight-edge appearing too low. This observation is facilitated by whitening the two inner broadsides of the straight-edges, these sides being those that are nearest to each other during the observation. The material for such whitening may be either white paper stuck to the tools, paint, soft whiting, or other white substance which is near.

Twist-finders differ from other straight-edges in being much thicker, and therefore too heavy for general use; but the planing and finishing are managed in a manner similar to that for other straight-edges. When it may be necessary to use two thin ones as twist-finders, they are made to stand with packing-blocks that are narrower than the straight-edges, in order that the observation may not be obstructed by the blocks being too high. The blocks are put close to the broadsides of the tools, to make them stand without other fastenings.

Straight-edges that are to be made into measures by marking lines upon their broadsides, are smoothly polished, and the marking is effected either with a straight-line dividing-machine, a machine having a fine micrometer screw, or by means of some other ordinary screw. Machinists who may be obliged to make a few measures instead of buying them, effect the marking with the screw of an ordinary lathe for screw cutting. With this object, the broadsides are first marked with three or four lines along the entire lengths of the tools, the lines being at different distances from each other to distinguish the shorter divisions from the longer ones. These long lines are made with a planing-machine and a sharp pointed springy tool; after which the straight-edge is fastened to a table of sufficient length attached to the side of the lathe farthest from the operator. The table being connected to the carriage allows the lathe-screw to move the straight-edge along the lathe. After the tool to be marked is properly adjusted to place the edge parallel to the length of the lathe, a sharp vee-tool, or other scriber is attached to a small tool-holder that may be connected for working by the lathe, or the tool-holder may be merely the usual upper slide of the rest that may be moved to and fro by the operator's hand. The screwing wheels for moving the tool may be only two - one having eighty teeth to be put on the lathe screw, and a small one having twenty teeth for a stud in the slot; these wheels being suitable for a screw having a half-inch step. It is not necessary to connect the wheels with the lathe spindle, the small wheel being rotated by an assistant during the process. If sixteenths are required on the tool, the small wheel is moved half a complete rotation at each mark, which advances the straightedge a sixteenth of an inch ; and if thirty second parts are desired, the pinion is moved only a quarter of a revolution each time. In order to cause the pinion to advance the proper number of teeth, a stop is put to the space between the two proper teeth, which are plainly marked at the commencement. If nicety is required in the marking, it is necessary to select a lathe whose screw is accurate, and with a screw-nut of great length, also having screwing wheels with teeth properly made. After all the marking is performed, the measure is adjusted by filing the extremities to the marks, if it is desirable to thus place the marks at the edges; if not, any required length is allowed to remain beyond the marks which constitute the measure.