A ceiling is an overhead interior surface that covers the upper limit of a room. It is not generally considered a structural element, but a finished surface concealing the underside of the floor or roof structure above. Ceilings are classified according to their appearance or construction. A cathedral ceiling is any tall ceiling area similar to those in a church. A dropped ceiling is one in which the finished surface is constructed anywhere from a few inches or centimetres to several feet or a few metres below the structure above it. This may be done for aesthetic purposes, such as achieving a desirable ceiling height; or practical purposes such as providing a space for HVAC or piping. An inverse of this would be a raised floor. A concave or barrel-shaped ceiling is curved or rounded upward, usually for visual or acoustical value, while a coffered ceiling is divided into a grid of recessed square or octagonal panels, also called a "lacunar ceiling". A cove ceiling uses a curved plaster transition between wall and ceiling; it is named for cove molding, a molding with a concave curve. A stretched ceiling (or stretch ceiling) uses a number of individual panels using material such as PVC fixed to a permieter rail. Ceilings have frequently been decorated with fresco painting, mosaic tiles and other surface treatments. While hard to execute (at least in place) a decorated ceiling has the advantage that it is largely protected from damage by fingers and dust. In the past, however, this was more than compensated for by the damage from smoke from candles or a fireplace. Many historic buildings have celebrated ceilings. Perhaps the most famous is the Sistine Chapel ceiling by Michelangelo. Painted ceiling, "Icarus", by Rainer Maria Latzke (c. 1986), Chateau Thal, Belgium The ceiling of Wells Cathedral, England Stretched ceiling Ceiling of Lotfollah Mosque, Iran Ceiling paintings of Einsiedeln Abbey in Switzerland Highly decorated Moorish-style ceiling in Agadir, Morocco demonstrative reconstruction of a Roman suspended ceiling in an Imperial palace of c. AD 306 at Trier Ceiling at the United States Library of Congress The interior of the Sistine Chapel in St. Peter's Basilica in Rome, showing the ceiling in relation to the other frescoes.