From Longman Dictionary of Contemporary Englishplumb lineˈplumb line noun [countable] TZa piece of string with a small heavy object tied to one end, used for measuring the depth of water, or for checking whether something such as a wall is built exactly upright
Examples from the Corpus
plumb line• If any walls appear to lean, check by going back to the nearest upper window and drop a plumb line down.• But again, as with the lintels, the union did not allow the bricklayers to use a plumb line. 