Square

Square describes two surfaces or edges meeting at exactly 90 degrees. It's both a state a piece can be in (a "square" corner, a cabinet checked and found "square") and a family of tools built to check or mark that angle -- speed squares, carpenter's squares, and machinist squares all serve this same core purpose at different scales of precision. A build that's out of square rarely fails obviously at first; it just quietly makes every later step -- doors, drawers, panels -- harder to fit right.