Place and fasten a straight edge on a smooth wall or surface, set the square on the straight edge and draw a line the length of the square on the wall. Reverse the square to the opposite side of the line and draw another line. If the lines run parallel, the square is true, if the lines divide, it is not true. Draw a line centered of the divided lines, and file the edge of the square until it reverses the same. (Fig. 69.)

Fig. 69