Transept

A transept is the transverse arm of a church that crosses the nave at a right angle, usually positioned between the nave and the apse, and its presence gives the building's overall floor plan the shape of a cross when viewed from above, with the nave as the long vertical shaft, the transept as the horizontal crossbar, and the apse extending the shaft's short remaining arm beyond the crossing. Where nave and transept intersect, the resulting square or rectangular space, called the crossing, is architecturally significant in its own right, frequently marked by a raised dome, tower or spire rising above it that has no equivalent anywhere else in the building, making the crossing tower one of the most recognizable exterior features of many cathedral silhouettes.

Functionally the transept's arms provide additional floor space branching off the main nave-to-altar axis, historically used for side altars, additional chapels, or simply as overflow space for a large congregation, and in churches built to a strict cruciform plan the transept arms are sometimes given their own smaller entrances, independent of the main west door, allowing separate points of access into the building. The transept's cross-shaped statement is deliberate rather than incidental: building a church whose very footprint traces the form of the cross became, from an early date, a way of expressing the building's Christian identity architecturally, independent of any decoration applied to its surfaces.

Transepts appear in Christian building as early as the fourth century, with Old Saint Peter's Basilica in Rome commonly cited among the earliest examples of a transverse hall inserted between the nave and the apse of an otherwise standard basilical plan, though early transepts of this period did not always rise to the full height of the nave or read as strongly as a cross shape from the exterior. The transept became a genuinely standard, structurally emphasized element of Western church architecture through the Romanesque period, when builders increasingly gave the crossing its own raised tower and treated the transept arms as full architectural statements in their own right rather than modest side extensions, a development that carried forward and intensified into the Gothic cathedrals of the twelfth and thirteenth centuries, where transept facades were sometimes given their own elaborate portals and, in French Gothic examples especially, their own large rose windows to match the west facade.

Eastern Christian church architecture developed the cross-shaped plan along a somewhat different path, favoring centrally planned domed churches, such as the cross-in-square plan standard in Byzantine architecture from the middle Byzantine period onward, in which the arms of the cross are typically shorter and more evenly balanced around a central dome than in the elongated Latin-cross plan characteristic of Western cathedrals.