A pier glass is a mirror

Mirror

A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...

 which is placed on a pier

Pier (architecture)

In architecture, a pier is an upright support for a superstructure, such as an arch or bridge. Sections of wall between openings function as piers. The simplest cross section of the pier is square, or rectangular, although other shapes are also common, such as the richly articulated piers of Donato...

, i.e. a wall between two window

Window

A window is a transparent or translucent opening in a wall or door that allows the passage of light and, if not closed or sealed, air and sound. Windows are usually glazed or covered in some other transparent or translucent material like float glass. Windows are held in place by frames, which...

s 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 large 18th century houses. An incorrect example is commonly given as the Hall of Mirrors at the Palace of Versailles

Palace of Versailles

The Palace of Versailles , or simply Versailles, is a royal château in Versailles in the Île-de-France region of France. In French it is the Château de Versailles....

 and Sanssouci Palace, yet the piers of these rooms are actually faced with pilasters, not pier glass. The mirrored glass is on the internal wall in a shape imitating the windows opposite. True pier glasses are found in the long galleries

Long gallery

Long gallery is an architectural term given to a long, narrow room, often with a high ceiling. In British architecture, long galleries were popular in Elizabethan and Jacobean houses. They were often located on the upper floor of the great houses of the time, and stretched across the entire...

of many 18th. c. houses.