In construction an architecture, a dropped ceiling, also referred to as a drop or suspended ceiling, is used as a secondary ceiling formed to conceal piping, wiring, or ductwork, into an area called the plenum. It consists of a grid-work of metal channels in the shape of an upside-down "T", suspended on wires from the overhead structure. These channels snap together in a regularly spaced pattern - typically a 2x2 or 2x4 foot grid in the US or 600 x 600 mm grid in Europe - and each cell is filled with lightweight "acoustic ceiling tiles" or "panels" which simply drop into the grid. Tiles can be selected with a variety of materials, including wood, metal, or mineral fibres, and can come in almost any color. Fluorescent light fixtures, air supply diffusers, and return air grilles of the same dimension, and incandescent lights, various electrical devices, and sprinkler heads are then installed into the grid as desired.