A plumb line is a weight on a string that shows true vertical. Drop a plumb line.
Gravity does the work, so it is right whatever the ground is doing. A plumb line is the oldest building tool there is and still cannot be improved on.
1nounplumb lineA weight on a string that shows true vertical.
“The plumb line showed the whole wall leaning by four centimetres.”
plumb line is a tool.
Plumb comes from Latin plumbum, lead, which is what the weight was made of.
Middle English
plumblyne
English, today
plumb line
From Middle English plumblyne, equivalent to plumb + line.
Etymology from Wiktionary, CC BY-SA
Practised in Tools & DIY