What does a level tell you about a surface?

A level indicates whether a surface is truly horizontal ('level') or truly vertical ('plumb'). It works using a sealed vial containing liquid and an air bubble; when the bubble sits centered between the marked lines, the surface is level or plumb. Levels are essential for hanging shelves, installing fixtures, laying tile, and framing so that surfaces are straight and even. A tape measure handles length, and a square checks 90-degree angles. Knowing the level's job — checking for horizontal and vertical alignment — is a common measuring-tool topic on the Shop subtest.