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 unto 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 , or ‘cross-dome church’. In French, it is an , ‘church with an inscribed cross’.
Architecture
Architectural form
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.
Although evidence for Byzantine domestic architecture is scant, it appears that the core unit of the cross-in-square church (nine bays divided by four columns) was also employed for the construction of halls within residential structures.
Liturgical use
The architectural articulation of the distinct spaces of a cross-in-square church corresponds to their distinct functions in the celebration of the liturgy. The narthex serves as an entrance hall, but also for special liturgical functions, such as baptism, and as an honored site of burial (often, as in the case of the Martorana in Palermo, for the founders of the church). The naos is the space where the congregation stands during the service. The sanctuary is reserved for the priests. The altar stands in the central bay, or bema, which is sometimes provided with a synthronon, or bench, where the clergy may sit. The prothesis is used for the preparation of the eucharist, and the diakonikon houses liturgical vestments and texts used in the celebration of the Liturgy.
Common variations
The architectural form and liturgical function described above correspond to the "classic" type of the cross-in-square church, which is exhibited by a number of significant monuments (for example, by the Myrelaion in Constantinople). However, this classic type represents only one of a number of possible variations on the cross-in-square form.
Particularly in later Byzantine architecture, the core of the cross-in-square plan could be augmented through the addition of peripheral structures. An example is provided by the Chora Church in Constantinople. The original 11th-century cross-in-square was expanded in the 14th century through the addition of a second narthex to the west (exonarthex, or outer narthex) and by a side chapel (parekklesion) to the south, used for burials. The ultimate plans of many other Byzantine churches resulted from a similar diachronic succession of additions about a central, cross-in-square, core; for example, Kalenderhane Camii in Constantinople, Çanlı Kilise in Cappadocia, and the Martorana in Palermo. One particularly common subsidiary structure, witnessed, for example, at Kalenderhane, the Chora Church, and the Martorana, was a bell-tower.