PLUMB

(STRAIGHT) [adjective] [after verb; not gradable] [specialized] - (esp. of something vertical) exactly straightWhen you hang a door, you need to make sure that it is both level and plumb.If something is out of plumb, it is not exactly straight.A plumb line is a piece of string with a weight fixed to one end, which is used either to test whether something that is vertical, such as a wall, is exactly straight, or to find the depth of water.