A pier glass or trumeau mirror is a mirror which is placed on a pier, i.e. a wall between two windows supporting an upper structure.
[1] It is therefore generally of a long and tall shape to fit the space.
This was a common decorating feature in the reception rooms of Neoclassical 18th-century houses.
It takes its name from the French word trumeau, which designates the space between windows.
A decorative carved or painted scene was the prominent characteristic, and could dominate the actual mirror.