Tetraconch

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.

[2] Later a different plan was developed, with a tetraconch main space completely encircled by an aisle, or ambulatory in the terminology used for Western churches,[3] as at the ruined mid-7th century Zvartnots Cathedral.

[4] The ruined so-called Cathedral of Bosra, of the early 6th century, is the earliest major Syrian tetraconch church,[5] though in Syria the type did not remain as popular as in the Caucasus.

The Mausoleum of Galla Placidia in Ravenna (425–30), world-famous for its mosaics, is almost a tetraconch, although there are short vaulted arms leading from the central space to each apse-end.