A plumb-bob or a plummet is a weight with a pointed tip on the bottom that is suspended from a string and used as a vertical reference line. This instrument has been used since the time of the ancient Egyptians by bricklayers, masons, and carpenters to ensure that their constructions are "plumb", or perfectly upright. It may also be used in surveying to sight a point on the ground that is not readily visible. Small plumb bobs are included in the kits of various instruments such as levels and theodolites. They are used to set the instrument exactly over a fixed datum marker, prior to taking fresh readings. Etymology of the name The plumb in the name plumb-bob comes from the fact that such tools were archetypically made of lead (Latin plumbum). The adjective plumb developed by extension. Use Up until the modern age, on most tall structures, plumb-bob

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