Also “plummet”. Symbol of the second “craft” / “symbolic” / “blue degree” and the emblem of the Junior Warden (2nd Warden) in “craft” lodges.
It often has a ‘vertical reference’, ‘acting upon the plumb’, keeping the ‘higher’ in mind.
Note that the plumb comes in different forms. Above you see a rope with a plumb on a wooden plank, but you also see just a rope with a plumb either or not partly rolled up or a rope with a plumb held by a person, etc. It can look quite a bit like the level.
An interesting variation can be found in the “Maitre Anglais” (‘English Master’) degree in the Bonseigneur collection where:
The square [represents] the Great Architect, the Level [represents] the Son, the Plumb rule [represents] the Holy spirit.
Duncan’s Masonic Ritual and Monitor (1866) by Malcolm C. Duncan