A spirit level is a tool with a bubble vial used to check horizontal and vertical alignment. The vial is a curved glass tube filled with liquid and a bubble. When the level is horizontal, the bubble sits between two lines. When it is not, the bubble moves to one side. The user adjusts the work until the bubble is centered. The spirit level is simple, accurate, and indispensable. It is one of the most common tools on a construction site.

Spirit levels come in several types. A torpedo level is small and used for tight spaces. A box level is long and used for framing and masonry. A line level is small and hangs on a string. A laser level projects a line or a plane of light. A digital level displays the angle in degrees. Each has its own accuracy and application. The bubble vial is the classic. The laser is the modern. Both are used. Both are useful.

Spirit levels must be calibrated. A level that is dropped or abused may lose its accuracy. To check a level, place it on a flat surface and note the bubble position. Turn it 180 degrees and check again. If the bubble is in the same position, the level is accurate. If it moves, the level is out of adjustment. A level can be adjusted with a screw. A level that cannot be adjusted must be replaced. The spirit level is a simple tool. It is the basis of alignment. Without it, the walls lean, the floors slope, and the doors do not close. The level is the truth. The bubble does not lie.