plumb rule vs plumb line

plumb rule and plumb line both are nouns.

Word	Noun	Adjective	Verb	Adverb
plumb rule	Yes	No	No	No
plumb line	Yes	No	No	No

As nouns, plumb line is a hypernym of plumb rule; that is, plumb line is a word with a broader meaning than plumb rule:

plumb rule: a plumb line attached to a narrow board
plumb line: a cord from which a metal weight is suspended pointing directly to the earth's center of gravity; used to determine the vertical from a given point

Other hypernyms of plumb rule include perpendicular.

plumb rule (noun)	plumb line (noun)
a plumb line attached to a narrow board	a cord from which a metal weight is suspended pointing directly to the earth's center of gravity; used to determine the vertical from a given point