What is a plumb? ?

A plumb is a weight, usually made of metal, that is attached to the end of a line and used to determine verticality or alignment. It is commonly used in construction and carpentry to ensure that walls, posts, and other structures are perfectly vertical.

In geometry, a plumb is a line that is perpendicular to a horizontal line. It is often used to refer to the vertical line that is used in construction to ensure that walls are built straight up and down.