A large number of articles can be lined by means of a primary straight line marked on one surface of each object, and extending along its entire length. The place of this straight line along the length of any article may be found by springy dividers, compasses, and callipers, the more convenient of these being callipers having points resembling Fig. 543. For marking a primary line upon a surface of a small rugged piece of plate, bar, or other object only a few inches across, it is only necessary to roughly place a straight-edge to the middle by merely looking to find the middle, and marking the line with a scriber ; but a large piece of work requires a little care, whether rough or smooth. To find the place of a primary with some pretension to accuracy, a calliper may therefore be adjusted, so that the distance between its two points is a little greater than half the width across the surface to be lined; the calliper is then put with one point in contact with one edge of the surface to be lined, and with the other point or foot extending rather more than half way across the surface; in this position the calliper is held with one hand, and a short arc marked through a little whiting on the surface. The next step is to shift the calliper to the opposite edge, and with the feet at the same distance apart, to scribe another short arc which shall intersect the first arc. The middle point between the two arcs is the centre of that particular portion of the surface, and it may be shown by marking a short straight line through the two points where the arcs intersect. To find the centre at any other particular part of the surface, the same means is adopted, with the difference of altering the distance between the calliper-points to suit the width of the surface, if it differs in width. By such centring, the place of a primary line along the middle of a key's taper side is easily indicated. If the surface of the small end of the key is two inches across, the calliper's feet are adjusted to one inch and a sixteenth from each other previous to scribing the arcs at the small end of the key. After these are marked, the calliper's feet are separated a short distance to suit the large end of the key; and if the distance across at this end is two and a half inches, the distance between the feet is one and five-sixteenths inches. With the points at this distance, two more short arcs are now made at the large end of the key, which shows the centre of this end, by the places of intersections of the arcs, as at the small ends; consequently, to indicate the centre line along the entire surface, a straight-edge is put to the four intersections of the arcs, and a line scribed, which is the primary required.