plumb rule

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A narrow strip of wood with a plumb line hanging from it, used to test verticality.

from The Century Dictionary.

• noun A narrow board with parallel edges having a straight line drawn through the middle, and a string carrying a metal weight attached at the upper end of the line. It is used by masons, bricklayers, carpenters, etc., for determining a vertical.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a plumb line attached to a narrow board