Levels

"Bubble levels," also known as spirit levels or bubble vials, are tools used to determine whether a surface is level or plumb (vertical). They contain a liquid-filled vial with an air bubble inside. When the bubble is centered between two reference lines in the vial, it indicates that the surface is level or plumb.

Using a Level

Bubble levels are useful to make sure a piece is straight when building things like field elements or making sure something is flat. To use a level, put it firmly against the object and move it until the bubble is in between the two lines.