A Plum Bobs is commonly referred to, a plumb line, consists of a pointed weight attached to a piece of string, and it uses gravity to show the user. It is a weight, usually with a pointed tip on the bottom, suspended from a string and used as a vertical reference line, or plumb-line. The plumb bob might not be the most exciting or innovative tool out there, but its undoubtedly one of the most useful. The bobs are as simple as they look, too, mainly consisting of a piece of string. The Plum Bobs are available in various types.