A pier glass is a mirror which is placed on a pier, i.e. a wall between two windows supporting an upper structure. It is therefore generally of a long and tall shape to fit the space. It may be as a hanging mirror or as mirrored glass affixed flush to the pier, in which case it is sometimes of the same shape and design as the windows themselves. This was a common decorating feature in the reception rooms of classical 18th-century houses.