TRIFORIUM, in architecture, a longitudinal passage or gallery of a church or other high veiled interior, the triforium floor being usually above a side aisle vault or ceiling. The usage of the term is loose ; by some it is applied to any second floor gallery opening on to a higher nave by means of arcades or colonnades, like the galleries occurring in some pagan Roman basilicas, or similar galleries in Byzantine churches. In other cases its usage is limited to an arcaded gallery above the side aisle of a mediaeval church and below the clerestory windows, thus occupying the height of the side aisle lean-to roof. The triforium became an integral part of interior church design during the Romanesque period, sometimes merely as a series of openings to light and ventilate the roof space, and sometimes as a complete open gal lery, often vaulted with a quarter circle vault, which transmitted the thrust of the main nave vault to the outside walls, as in the church of S. Cernin at Toulouse (begun 1096). With the develop ment of articulated Gothic vaulting in France the usage became confused. At times there was a complete vaulted gallery over the side aisle, which created a lower triforium; in the height of the lean-to roof over this a second, minor triforium was developed, as in Laon cathedral (I16o-12o5), and in Noyon (II50–I200).
In Notre Dame at Paris (1163-96) the upper triforium was originally represented by a series of simple, round openings with rudimentary tracery opening to the roof space. The change in Notre Dame, in the rebuilding (between 124o-58), by which the aisle roofs were flattened, the clerestorey windows heightened and the circular openings eliminated, shows the 13th century dissatis faction with the old four stage composition, and the upper vaulted gallery is found in few churches of the developed Rayonnant pe riod. Chartres cathedral (1194-1212), Reims cathedral (begun
122o) and Amiens (122o-47) all show triforia of little relative height, but of rich arcading and, in the last, the central mullion of the clerestorey tracery is carried down through the triforium ar cading, so as to bind clerestorey and triforium into one composi tion. In the apse of Amiens, moreover, the flatness of the side aisle roofs allows windows to be placed behind the triforium ar cading, which thus becomes, as it were, merely a decorated base to the slim clerestorey above. This development reaches its climax in the church of S. Urbain at Troyes (1262-8o), in which the tri forium gallery completely disappears as a separate motif, although it is faintly recalled by the tracery of the clerestorey windows.
In the Flamboyant period the entire omission of the triforium becomes the rule. The English love of horizontal lines shows in a great development of the triforium gallery as an important dec orative element in church interiors. In both Norman and Gothic work it is relatively much higher than in similar work in France, often almost equalling the pier arcades. It frequently receives the richest possible treatment, by the use of both tracery and sculp ture. The richest and most characteristic example is the angel choir at Lincoln (completed 1282). In the 15th century the tendency toward height of pier arcade and clerestorey window, so clearly shown in Perpendicular work generally, leads to the marked reduction in importance and the final disappearance of the triforium. (See BYZANTINE AND ROMANESQUE ARCHITECTURE;