Spirit level

A spirit level is an instrument with a liquid-filled vial and an air bubble that indicates whether a surface is horizontal or vertical.

Bubble level vial: A sealed, curved vial partly filled with liquid, leaving a gas bubble that moves with the vial’s orientation. Its curved interior makes the bubble settle at the vial’s highest point.
Horizontal plane: A plane perpendicular to the local direction of gravity. A spirit level’s horizontal check tests whether a surface lies in this orientation.
Carpentry: The craft of cutting, shaping, and joining wood to make structures and objects. Carpenters use levels to check framing, shelves, and other installed work.
Inclinometer: An instrument that measures the angle of slope or tilt relative to a reference direction. It reports angular inclination, while a basic spirit level primarily indicates level or plumb.
Hydrostatics: The study of fluids at rest and the forces and pressures within them. The vial’s liquid and bubble respond to gravity as a fluid system.
Verticality: The condition of being aligned with the local direction of gravity. Turning a level on its side tests whether a surface is vertical.
Masonry: Construction using units such as stone, brick, or concrete block joined with mortar. Masons check courses and walls for level and plumb as they build.
Electronic level: An instrument that uses electronic sensors to measure and display an object’s inclination. It provides a numerical angle rather than relying on visual bubble position.
Buoyancy: The upward force exerted by a fluid on an immersed object. The bubble rises through the denser vial liquid toward the vial’s high point.
Plumb line: A cord carrying a suspended weight that indicates the local vertical direction. It provides a simple gravity-based reference comparable to a level’s vertical vial.

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