A carpenter’s level is typically made of wood, aluminum, or magnesium, and is rectangular in shape, from 9 to 24 inches in length. It might have one or more true surfaces having a glass vial in the center of each surface. The glass vial right in the center of a horizontal surface and parallel with it is used in leveling; a glass vial in the end of a level, perpendicular to the true surface, is used to check the plumb or vertical trueness.
To be sure that the level is correctly adjusted, lay it on any surface and note the position of the bubble in the vial. Reverse the level end-for-end. The bubble must be in the same relative position as it was initially. When it is not, the level is out of adjustment and shouldn’t be used till suitable adjustments are done. Ordinarily adjusting screws that hold the vial in place, can be tightened to give a correction of the reading. A similar testing and adjusting process must be used for checking the plumb.
To check whether a surface is level or plumb, put the level on the surface to be leveled or plumbed, while holding the level with the hand. When the position of the bubble isn’t in the center of the vial, the surface is not level or plumb. The bubble always moves toward the higher side.
The level is a fragile instrument, easily damaged, and great care should be taken so that it is not misused. It should be kept in a toolbox and secure from the danger of being broken by some other tools in the box.
Some spirit levels integrate two or more glass vials so that measuring could be done from two directions at the same time—one or two horizontal vials and the vertical vial for “plumbness.” A few instruments have vials in pairs as a check against one another, and as a safety margin against breakage. Like with all tools, the accuracy of the level depends upon its material and workmanship. Wood levels could sometimes warp. Aluminum and magnesium levels are more precise. A variant of the carpenter’s level in usage is a “line level” that can be hooked onto a line from two points and check level for accuracy.