Level is an instrument used by carpenters, plumbers, and others to determine if a surface is perfectly horizontal. Most levels consist of a small, arched glass tube set in a bar of wood or metal. The tube is marked at its center, which is its highest point. It contains alcohol or another liquid and a bubble of air. When a level is placed lengthwise on a perfectly horizontal surface, the bubble rests at the center of the tube. If the surface being checked is tilted, the bubble rises to the high end of the tube. Some levels have additional tubes that are set at right angles to the length of the tool. These tubes enable workers to tell if a surface is plumb (vertical).