A plumb bob is a weight with a pointed tip that is attached to a string. The force of gravity
causes the weighted line to hang vertically, or plumb. Plumb bobs come in
different weights; the most common are 6, 8, and 12 ounces.
To mark a vertical line on a wall:
Hang a plumb bob from a chalk-coated string almost against the wall
(ideally with a tiny gap). Secure the string at the top, near the wall,
with a nail.
Steady the weight by hand until it stops swinging and twirling.
Leave the weight to find its final position.
Hold the weight against the wall and snap the string against the wall
to create a vertical chalk line.
Remove the chalk line when no longer needed.