Plumb-bob
A plumb-bob is a weight with a pointed tip on the bottom that is suspended from a string, and is used as a reference line that is perpendicular to the ground. This instrument has been used since the time of the ancient Egyptians by bricklayers, masons, and carpenters to ensure that their constructions are "square", or perfectly upright. It may also be used in surveying to sight a point on the ground that is not readily visible.
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