Cross-in-square

A cross-in-square or crossed-dome plan was the dominant architectural form of middle- and late-period Byzantine churches. It featured a square centre with an internal structure shaped like a cross, topped by a dome.

The first cross-in-square churches were probably built in the late 8th century, and the form has remained in use throughout the Orthodox world until the present day. In the West, Donato Bramante's first design (1506) for St. Peter's Basilica was a centrally planned cross-in-square under a dome and four subsidiary domes.

In German, such a church is a Kreuzkuppelkirche, or ‘cross-dome church’. In French, it is an église à croix inscrite, ‘church with an inscribed cross’.

A cross-in-square church is centered around a quadratic naos (the ‘square’) which is divided by four columns or piers into nine bays (divisions of space). The inner five divisions form the shape of a quincunx (the ‘cross’). The central bay is usually larger than the other eight, and is crowned by a dome which rests on the columns. The four rectangular bays that directly adjoin this central bay are usually covered by barrel vaults; these are the arms of the "cross" which is inscribed within the "square" of the naos. The four remaining bays in the corner are usually groin-vaulted. The spatial hierarchy of the three types of bay, from the largest central bay to the smallest corner bays, is mirrored in the elevation of the building; the domed central bay is taller than the cross arms, which are in turn taller than the corner bays.

To the west of the naos stands the narthex, or entrance hall, usually formed by the addition of three bays to the westernmost bays of the naos. To the east stands the bema, or sanctuary, often separated from the naos by templon or, in later churches, by an iconostasis. The sanctuary is usually formed by three additional bays adjoining the easternmost bays of the naos, each of which terminates in an apse crowned by a conch (half-dome). The central apse is larger than those to the north and south. The term bema is sometimes reserved for the central area, while the northern section is known as the prothesis and the southern as the diakonikon.

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