In architecture, curtain wall has two meanings. The earlier meaning is the wall that surrounds a castle. The later meaning is a wall that functions like a curtain on a steel frame building. The idea is that since the steel frame functions as the support the wall just needs to cover that frame. This is one of several skelton and skin types. Many of these structures are covered in glass, but there are others that use other materials as well. The important distinction is that the curtain is not responsible for any structural support.