The plumb bob is one of the oldest tools in the world. The Egyptians used them for building. The key in a building construction is keeping things square to the ground. Perfectly horizontal or vertical as the case may be. The key to the plumb bob is that the point at the end of the bob is directly in line with the string. For example, when building a brick wall, one mounts the plumb bob above the area being bricked so that the point is over a reference point on the ground. This gives the bricklayer a perfect, taut, vertical string to guarantee that the wall of bricks will be all the same distance from the string and hence be vertically straight. On a construction site, taut strings are routinely used as references and are even chalked and snapped against a surface to leave an accurate line. For the last two centuries or so, plumb bobs have been typically made of brass or bronze with a steel tip. The top screws off so that the string can be run through a hole in the center of the top ensuring that the string is aligned with the tip.

A plumb-bob or a plummet is a weight, usually with a pointed tip on the bottom, that is suspended from a string and used as a vertical reference line, or plumb-line. It is essentially the y-axisequivalent of a "water level".

The instrument has been used since at least the time of ancient Egypt^[1] to ensure that constructions are "plumb", or vertical. It is also used in surveying to establish the nadir with respect to gravity of a point in space. They are used with a variety of instruments (including levels, theodolites, and steel tapes) to set the instrument exactly over a fixed survey marker, or to transcribe positions onto the ground for placing a marker.^{[2]  [Source]}