Plumb, Level, and Square
The three fundamental alignment references in construction: plumb means perfectly vertical, level means perfectly horizontal, and square means two surfaces meet at exactly 90 degrees.
In Detail
Plumb is checked with a spirit level or plumb bob held against the vertical surface. Level is checked with a spirit level on the horizontal surface. Square is checked with a framing square or the 3-4-5 triangle method (measure 3 feet on one side, 4 feet on the other — the diagonal should be exactly 5 feet). Buildings that are out of plumb, level, or square compound errors throughout construction, making doors, trim, and finishes impossible to install properly.