Cross-squares are represented by Fig. 291. Such a tool is made of two principal pieces, one of which is the thin blade that slides through a slot which is formed in the middle of the pedestal or shoulder-piece. This slot is of great length, because it is necessary to prevent the blade wearing and getting out of its position at right angles to the shoulder-piece. A small screw having a milled head is also shown in the Figure, for fixing the blade at any required length from the shoulder, indicated by S. On the blade are marked inches and parts, or millimetres, according to the intended use of the tool. A cross-square is used for the same purposes as tee-squares, and also for measuring depth of holes, recesses, slots, key-ways, and similar openings. Cross-squares are never used to accurately adjust surfaces to right angles with each other, because the blades cannot be adjusted to right angles with their shoulder-pieces so as to remain in such condition for any great length of time.