The Collaborative International Dictionary
Plumb rule
Plumb \Plumb\ (pl[u^]m), n. [F. plomb, L. plumbum lead, a leaden ball or bullet; cf. Gr. mo`lybos, mo`libos, mo`lybdos. Cf. Plummet, Plunge.] A little mass or weight of lead, or the like, attached to a line, and used by builders, etc., to indicate a vertical direction; a plummet; a plumb bob. See Plumb line, below. Plumb bob. See Bob, 4. Plumb joint, in sheet-metal work, a lap joint, fastened by solder. Plumb level. See under Level. Plumb line.
The cord by which a plumb bob is suspended; a plummet.
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A line directed to the center of gravity of the earth.
Plumb rule, a narrow board with a plumb line, used by builders and carpenters.
WordNet
plumb rule
n. a plumb line attached to a narrow board