So what is a plumb line?
Here’s a pretty good definition that I found.
“A plumb line, also called a plummet, is a cord with a non-magnetic weight attached to one end. When the cord is held in such a way that the weight can dangle freely, an exact vertical can be determined. Painters and carpenters use plumb lines to keep their work straight. It is difficult, while in the middle of a project, to determine a true horizontal or vertical line without an objective measuring tool, so a plumb line is employed. A plumb line applies the law of gravity to find right angles, to indicate the most direct route from top to bottom, and to keep things plumb. A plumb line doesn’t change or move with the whims of the carpenter. It remains true, and all work must line up with it or risk being crooked.”