a. Common Level. The common level (Figure 2-40) is used for both guiding and testing when bringing work to a horizontal or vertical position. The level has a long rectangular body of wood or metal, which has a built-in glass spirit tube on its side and near the end. Each tube contains a nonfreezing liquid with a small air bubble free to move within the tube. The side and end tubes are at right angles to each other. When you center the bubble of the side tube with the hairline, the level is horizontal; when you center the bubble of the end tube with the hairline, the level is vertical. By holding the level against a surface to be checked, you can determine whether the surface is truly level (or plumb). Levels should be hung up when not in use.

b. Plumb Bob. A plumb bob is made of metal, usually brass. It usually has a screw-type cap with a hole in the center. A string or plumb line is inserted through the hole and fastened inside. The bottom end has a point in direct line with the hole in the cap (Figure 2-41). The string is absolutely perpendicular to the horizontal when the plumb bob is suspended on it. It can be used for the same purpose as the plumb glass on a level; however, the plumb bob is not accurate when used in the wind.

c. Chalk Line. This is a strong, lightweight cord used to make a straight line between two widely separated points. To snap a straight line, rub chalk on a cord held taut between two points. Then pull the cord straight up from the center and release it, to allow it to spring back into place. Chalk lines come in metal or plastic cases. Figure 2-42 shows how to snap a chalk line.