Abstract: A carpenter's square having a first and second leg at right angles to each other, and a third leg extending between the first and second legs. A magnetic strip on the first leg for attaching the tool to a metallic surface to be plumbed. A first notch in the third leg a first distance from the first leg and a second notch in the third leg a second distance from the first leg so that a plumb string can be threaded through the first and second notches and thereby held in place to support a plumb bob to allow a plumb reading to be taken without the need for manually supporting the plumb bob.