A dropped or false ceiling is a secondary ceiling installed below the main structural ceiling. It consists of a grid framework made of metal channels suspended from overhead wires. The grid forms a pattern of cells that hold lightweight ceiling tiles. Common tile materials include mineral fibers, metal, plastic, and wood. The space above the false ceiling, called the plenum, is often used to hide wiring, ductwork, and pipes. False ceilings provide acoustic control and conceal the underside of the floor above. Different types of false ceiling systems include exposed grid, concealed grid, and semi-concealed grid styles.