The curtain wall, one of architecture’s most provocative metaphors, is surprisingly difficult to pin down with a precise definition. Because it can be examined from multiple perspectives—in terms of functional relationships, as an aesthetic object, or as a massproduced system available within the construction marketplace—some ambiguity is both inevitable and provocative. In the first case, the curtain wall is defined in terms of its functional relationship to the building’s structure. It then refers to the cladding, or enclosure, of a building as something both separate from and attached to the building’s skeletal framework. Where load-bearing walls provide both structure and enclosure, there can be no curtain wall. However, difficulties emerge within this first definition when the question of “in-fill” is considered: are conventional windows (or other in-fill material), when fixed inside the boundaries of a structural frame, considered to represent curtain-wall construction? Such cons...