Mason's LEVEL, is composed of three rules, so joined as to form an isosceles-rectangle, somewhat like a Roman A; at the vertex whereof is fastened a thread, from which hangs a plummet, that passes over a fiducial line, marked in the middle of the base, when the thing to which the level is applied is horizontal; but declines from the mark, when the thing is lower on the one side than on the other.