Plumb is the verticality of something. This is tested by means of a plumb line which is usually a piece of string with a plumb bob at the end, this is any weight but used to be lead (plumbum is an obsolete word for lead). A plumb line is also used for determining the depth of water. A plumb rule is a plumb line attached to a narrow board, this was used by surveyors before the days of the dumpy level.