**PLUMB-RULE**, or **Plumb-line**, an instrument used by carpenters, masons, and others, in order to judge whether walls or beams be upright planes, horizontal, or the like. It is so called from a piece of lead fastened to the end of a cord, which usually constitutes this instrument. Sometimes the string descends along a wooden ruler, raised perpendicularly on another; in which case it becomes a level.