What is the purpose of a plumb bob?

The purpose of a plumb bob is to maintain vertical alignment. A plumb bob is a simple tool, typically a weight suspended from a string, that uses the force of gravity to indicate a true vertical line. When the bob is allowed to hang freely, the string will align perfectly with the vertical direction, making it essential in construction and surveying for ensuring that structures are erected plumb (vertically straight).

This tool is particularly useful in various applications, such as establishing the vertical alignment of walls, posts, and other structures, as well as for checking the uprightness of existing structures. Its design relies solely on gravity, which provides a reliable reference point for verticality, making it an indispensable instrument in both surveying and building construction.