A plumb bob is a weight on a string used to establish a true vertical line. It is one of the oldest tools in construction. The weight is usually brass or steel, shaped like a cone or a cylinder. The string is attached to the top. Gravity pulls the weight down, and the string becomes vertical. The user marks the line or aligns the work to it. The plumb bob is simple, accurate, and reliable. It does not need batteries or calibration. It works everywhere.

The plumb bob is used in many trades. A mason uses it to check that a wall is plumb. A carpenter uses it to align a door frame. A surveyor uses it to transfer a point from one level to another. A foundation contractor uses it to set anchor bolts. The plumb bob is often used with a level and a chalk line. The level checks horizontal. The plumb bob checks vertical. Together they define the two axes of construction.

The plumb bob has been largely replaced by the laser level and the digital level. But it is still used. It is more accurate than a spirit level for tall vertical elements. It is not affected by wind if it is sheltered. It is cheap and durable. Every carpenter should own one. It is a reminder that gravity is the original level. The plumb bob is a small tool. It establishes the vertical. Without the vertical, the building leans. The plumb bob is the foundation of alignment.