A spirit level is a tool used to determine whether a surface is horizontal (level) or vertical (plumb). It typically consists of a sealed tube filled with a liquid, usually colored alcohol or water, and an air bubble. The tube is mounted in a frame, often made of wood or metal. When the spirit level is placed on a surface, the bubble will move within the tube. If the surface is perfectly level, the bubble will center itself between two marked lines on the tube.
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