plumb rule(n = noun.artifact) - a plumb line attached to a narrow board;
is a kind of perpendicular, plumb line
algorithmic rule, as a rule, best evidence rule, board rule, carpenter's rule, exclusionary rule, foot rule, gag rule, golden rule, grammatical rule, ground rule, hearsay rule, heuristic rule, home rule, linguistic rule, majority rule, miranda rule, mob rule, morphological rule, naegele's rule, one-man rule, parol evidence rule, phase rule, plumb, plumb bob, plumb level, plumb line, procrustean rule, rule, rule book, rule in, rule of cy pres, rule of evidence, rule of grammar, rule of law, rule of morphology, rule of thumb, rule out, slide rule, work to rule, working rule