Which tool is used to ensure that a surface is level?

A spirit level is used to ensure that a surface is level by using the principle of gravity. It consists of a liquid-filled vial with an air bubble inside. When the bubble is centered between the marked lines on the vial, it indicates that the surface is perfectly horizontal. This tool is essential in carpentry and construction for tasks such as installing cabinets, shelving, and countertops to ensure they are even and properly aligned.

In contrast, a square is primarily used for checking right angles, a plumb bob is utilized to establish a vertical reference point, and a measuring tape is designed for measuring lengths or distances but does not assess the levelness of a surface. Each of these tools serves a specific function in the carpentry trade, but the spirit level is specifically designed for leveling tasks.