A tool used to establish a true vertical line. It's often used in construction and surveying. The plumb bob in the image appears to be made of brass, and it comes in various weights, as indicated on the packaging. A plumb bob is a pointed weight suspended from a string to indicate a vertical direction. It is used to ensure structures are level and upright. The brass cap on top stores an extra tip. The packaging also mentions that the plumb bob is precision-machined and has a replaceable hardened steel point for durability.