Plumb Line Bob

Plumb line is a tool used widely by carpenters to build straight edges.

wikipedia: A plumb bob or a plummet is a weight, usually with a pointed tip on the bottom, that is suspended from a string and used as a vertical reference line, or plumb-line. It is essentially the vertical equivalent of a "water level".

EDIT: The bob is now hollow and can be filled with sand or pebbles or any other heavy material to make it more stable.