Place the plumb on a smooth wall, hold the plumb to center the bulb and draw a line the length of the plumb stock, reverse the plumb to the opposite side of the line and draw a line the length of the plumb, if the lines are parallel, the plumb is true; if the lines divide at one end, it is not true. Make a mark 1/2 the distance between the divided lines and draw a line from this mark to the intersection of the two lines, then adjust the plumb to the center, or plumb line. (Fig. 71.)