Concise Bible Dictionary:
Plumbline
The simple contrivance of a lump of lead, a stone, or other weight attached to a string, for testing whether a building or other erection is perpendicular. It is used symbolically for the exactness with which judgment was brought upon Israel. Israel had been built up by God as a wall with a plumbline, and with a plumbline it should be destroyed (Amos 7:7-8; compare 2 Kings 21:13; Isa. 28:17). In Zechariah 4:10, although it was a day of small things when the temple was rebuilt, the plummet was in the hands of Zerubbabel, and the Lord of hosts was supporting him.
Plumbline – Ivory – Roman and Greek Art