A plumb bob is an old fashioned tool that is used to measure the vertical alignment of doors, windows, walls, studs and many other projects. It is a simple tool that is basically a heavy brass bulb with a pointed end that is attached to the end of a string. They are very easy to use and incredibly accurate in how they take measurements, find center points, and align things in a vertical axis.
Step 1 - Measure Masonry String
Place the hook end of the tape measure on the extreme edge of the area where you want to find the center point. Mark it at the three inch mark. Repeat the process on other end.
Step 2 - Secure Nail
Drive a nail into the section a few inches. This will give you somewhere to attach the plumb bob so it can hang.
Step 3 - Attach Plumb Bob
Hook the end of the string to the nail. Let the plumb bob hang and secure the string so that the plumb bob is 1/4 inch off the masonry surface. Hold the plumb bob still and let it hang freely.
Step 4 - Mark Center
Use the tape measure and place the hook under the tip of the bob. Stretch it out to the right and locate the center between the two marks.