The plumb is usually a lead weight, but it can be made of any other metal, cylindrical or prismatic in shape "Prism (geometry)"), the lower part conical in shape "Cone (geometry)"), which by means of the rope from which it hangs marks a vertical line; in fact, the vertical is defined by this instrument.

A nautical probe, used to measure the depth of water, is also given this name. Both in architecture and in nautical, it is a very important instrument.

The plumb line uses gravity to establish what is truly “vertical”; In construction it is used to know if a wall is vertical and perpendicular to verify the stability and weight distribution of the structure, in addition to remaining correctly as the level plane of a work.

This is of great importance, since if a wall is not plumbed it would give the possibility of the structure collapsing due to a decrease in its resistance.

700g weight.

In Freemasonry, the plumb line is the badge of the inspector or second watchman, a symbol of equality.[3]​.