Fig. 1468.
If we have two squares that are trued and have their edges parallel, we may test them for being at a right angle by trying them together as in [Figs. 1466] and [1467], in which a, b, are the two squares which, having their back edges pressed firmly together (when quite clean), must coincide along the blade edges; this being so we may place them on a truly surfaced plate as shown in [Fig. 1468], in which s is one square and s′ the other, p being the surface plate. Any want of truth in the right angle will be shown doubled in amount by a want of coincidence of the blade edges.
Fig. 1469.
For some purposes, as for marking out work on a surface plate, it is better that the square be formed of a single piece having the back and blade of equal thickness, as shown in [Fig. 1469], which represents a side and edge view of an L-and T-square respectively.
Fig. 1470.
For angles other than a right angle we have the bevel or bevel square (as it is sometimes called), shown in [Fig. 1470], a representing the stock or back, and b the blade, the latter being provided with a slot so that it may be extended to any required distance (within its scope) on either side of the stock. c is the rivet, which is made sufficiently tight to permit of the movement by hand of the blade, and yet it must hold firmly enough to be used without moving in the stock. Instead of the rivet c, however, a thumb-screw and nut may be employed, in which case, after the blade is set to the required angle, it may be locked in the stock by the thumb-screw.