Tetraconch

A tetraconch, from the Greek for "four shells", is a building, usually a church or other religious building, with four apses, one in each direction, usually of equal size. The basic ground plan of the building is therefore a Greek cross. They are most common in Byzantine, and related schools such as Armenian and Georgian architecture. It has been argued that they were developed in these areas or Syria, and the issue is a matter of contention between the two nations in the Caucasus. Apart from churches, the form is suitable for a mausoleum or baptistery. Normally there will be a higher central dome over the central space.

The Basilica of San Lorenzo, Milan (370) is possibly the first example of a grander type, the "aisled tetraconch", with an outer ambulatory. In middle Byzantine architecture, the cross-in-square plan was developed, essentially filling out the tetraconch to form a square-ish exterior. Either of these types may also be described less precisely as "cross-domed". In these types the semi-dome of the apse usually starts directly from the central domed space.