One of the most challenging aspects of framing work is making sure that everything is properly aligned. Plumb bobs have been around since Egyptian times to ensure a true vertical line is being achieved. In layman's terms, a plumb bob helps you make sure that your top is directly above your bottom. They're especially helpful in making sure doorways are straight. You can pick up a more sophisticated digital version, or save some money and use the original. Just make sure there's no breeze that will affect its accuracy.