The origin of the term is unknown but probably derived from thoroughfarum, as it was used as a passage from one end of the building to the other. The derivation from Latin tres, three, and foris, door, entrance, does not seem appropriate.
The earliest examples are those in the pagan basilicas, where it constituted an upper galley for conversation and lusiness; in the early Christian basilicas it was usually reserved for women, and the same applied to those in the Greek Byzantine Church.
In Romanesque and Gothic buildings it is either a spacious gallery over the side aisles or is reduced to a simple passage in the thickness of the walls; in either case it forms an important architectural division in the nave of the cathedral or church, and being of less height gives more importance to the ground storey or nave arcade. In consequence of its less height it was usually divided into two arches, which were again subdivided into two smaller arches and these subdivisions increased the scale.
On account of the richness of its mouldings and carved ornament in the sculpture introduced in the spandrils, it became the most highly decorated feature of the interior, the triforium at Lincoln being one of the most beautiful compositions of Gothic architecture. Even when reduced to a simple passage it was always a highly enriched feature. In the 15th-century churches in England, when the roof over the aisles was comparatively flat, more height being required for the clerestory windows, the triforium was dispensed with altogether. In the great cathedrals and abbeys the triforium was often occupied by persons who came to witness various ceremonies, and in early days was probably utilized by the monks and clergy for work connected with the church.
From the constructive point of view, the triforium sometimes served very important functions, as under its roof exist arches and vaults carried from the nave to the outer wall, to which they transmitted the thrust of the nave vault; even when the flying buttress was frankly adopted by the Gothic architect and emphasized by its architectural design as an important feature, other cross arches were introduced under the roof to strengthen it.
See also: Cathedral architecture