Spirit Level

A spirit level is a tool that may be used to determine whether or not a surface is level. Framed in either plastic, wood, or metal, it holds a glass tube filled with liquid and has an air bubble in the middle of the tube. A level, sometimes known as a bubble level, can also be referred to as a spirit level. This is due to the fact that the liquid contained within the vial is a mineral spirit. The hue of the mineral spirit solution ranges from yellowish to greenish since it contains chemicals that prevent the mineral spirit from fading when exposed to UV light and colorant. 

The liquid (and bubble) tube give the illusion of being cylindrical, but it isn't. It's slightly concave in shape. If the level is perfectly horizontal, the bubble will go toward the tube's center since its top will be somewhat higher there than at either end. The peak will be somewhat off-center if the level is not perfectly vertical. These vials have a bubble in them because they were only partially filled with liquid, typically a colorful spirit or alcohol. They're curved ever-so-slightly upward, allowing the bubble to rest comfortably in the middle at the highest point. The bubble moves away from the designated center at relatively small angles.

Professional videographers, surveyors, millwrights, stonemasons, and other members of the building trades all use spirit levels, as do professional photographers. Two banana-shaped, curved glass vials were used at each viewing point on the original spirit levels, making their use more involved.

A spirit level may be used for both horizontal and vertical leveling on a wide range of home improvement tasks. There are spirit levels of varying sizes and shapes for use in determining whether or not a surface is horizontally or vertically level. Egyptian builders would use a plumb line attached to a "A" frame, and Romans would fill tanks with water to use as a crude spirit level, to make sure all of their stones were perfectly level.