A spirit levelis a tool used to determine both how level a horizontal surface is and how plumb a vertical surface is. Most spirit levels are made of tough, lightweight metals such as magnesium or aluminum. The spirit level has two or three slightly curved glass tubes or vials, almost filled with ethyl alcohol (used because of its low freezing point), which measure either plumb (vertical) or level (horizontal). The amount of liquid in each vial is not enough to fill it, so there is always a bubble in the vial. When the bubble is centered between the lines on the vial, the surface is either level (exactly horizontal) or plumb (exactly vertical), depending on the vial.
The spirit level is the most commonly used level in the construction trade. Different types are used by carpenters, stone masons, bricklayers, other building trades workers, surveyors, and millwrights. Other types of levels include electronic levels and laser levels.