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.