plumb line

noun
1.
a cord with a lead bob attached to one end, used to determine perpendicularity, the depth of water, etc.
Compare plumb (def 1).
2.
plumb rule.
Origin
1530-40
Examples from the web for plumb line
  • The steel tapeline must be attached firmly to a flat surface with clear strapping tape, in a straight, plumb line.
  • Mark a plumb line down the center of one end and a horizontal line perpendicular to it.
  • Think of it as a plumb line from the ears to the knees.
British Dictionary definitions for plumb line

plumb line

noun
1.
a string with a metal weight at one end that, when suspended, points directly towards the earth's centre of gravity and so is used to determine verticality, the depth of water, etc
2.
another name for plumb rule
plumb line in Science
plumb line
  (plŭm)   
A line regarded as directed exactly toward the Earth's center of gravity.