a plumb line attached to a narrow board, used by builders, surveyors, etc
plumb rule in American English
a narrow board equipped with a plumb line and bob, used by carpenters, masons, etc.
plumb rule in American English
noun
a device for determining perpendicularity, consisting of a narrow board with a plumb line and bob suspended from an upper edge
Word origin
[1350–1400; ME plomreule]This word is first recorded in the period 1350–1400. Other words that entered Englishat around the same time include: absolute, fix, negative, range, screen