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A triforium is an interior gallery, opening onto the tall central space of a building at an upper level. In a church, it opens onto the nave from above the side aisles; it may occur at the level of the clerestory windows, or it may be located as a separate level below the clerestory. Masonry triforia are generally vaulted and separated from the central space by arcades. Early triforia were often wide and spacious, but later ones tend to be a shallow, within the thickness of an inner wall, and may be blind arcades, not be wide enough to walk along. The outer wall of the triforium may itself have windows (glazed or unglazed openings), or it may be solid stone. A narrow triforium may also be called a "blind-storey", and looks like a row of window frames.
The origin of the term is unknown but may be derived from Latin trans, "through", and foratum, "bored, drilled, cut", as it was a hollow passageway from one end of the building to the other, as suggested by the Trésor de la langue française. A derivation from Latin tres, three, and foris, door, entrance, might also be possible as in this passage the thoroughfares and doors were often in triangle shape as can be imagined from the triangular shape of this area, although the Lewis and Short Latin dictionary does not quote these words in combination, only separately. The triangle shape comes from the sloping roof, as can be seen in the picture on the right between the two arrows.
The earliest examples of triforia are those in the pagan basilicas, where a triforium constituted an upper gallery for conversation and business; in the early Christian basilicas such a passageway was usually reserved for women, and the same applied to those in the Eastern Orthodox 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 lesser height its bay was usually divided into two arches, which were again subdivided into two smaller arches and these subdivisions increased the apparent scale of the aisle below and the clerestory above.
On account of the richness of its mouldings and carved ornament in the sculpture introduced in the spandrels, it became the most highly decorated feature of the interior, the triforium at Lincoln being one of the most beautiful compositions of English 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 utilised by the monks and clergy for work connected with the church.
The triforium sometimes served structural functions, as under its roof are arches and vaults which carry thrust from the nave to the outer wall. 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.
The Roman forum; note people looking out from the triforium of the Basilica Julia, above left. The arches on both sides of the basilica's triforium were unglazed.
The Norman Malmesbury Abbey, showing the triforium, with its rounded arches and chevron mouldings, each arch supported by four small arches on columns. This triforium contains an unusual projecting watching-loft. There is also another passage above, at the base of the clerestory windows. Malmesbury, Wiltshire, England
Interior view of Notre-Dame's nave wall, showing (top to bottom) clerestory window, triforium, and side aisle openings.
Triforium of the K?l?ç Ali Pasha Mosque
Triforium in a shopping mall in Dalian, a public space more similar in purpose to the Roman basilicas