Appendices - Strolling Through Istanbul: The Classic Guide to the City - page 476

the church of SS. Sergius and Bacchus built by Justinian in 527. In
form it is an octagon inscribed in a rectangle with a projecting apse
and a large central dome. Between the eight piers which support the
dome, there are pairs of columns on the ground floor and gallery
level, thus making continuous ambulatories except in front of the
apse. SS. Sergius and Bacchus closely resembles the contemporary
church of St. Vitale at Ravenna.
The 250 years after Justinian (roughly 600 to 850) were a period
of decline and confusion, unfavourable to the arts. When architecture
began to revive in the ninth century, a new type of church building
came into vogue, generally known as the cross-domed church. In this
type a central dome is surrounded in the axes of the building by four
long barrel vaults resting on four strong corner piers, thus forming
an internal cross; on three sides are aisles and galleries, so that the
exterior is rectangular. At the east end the wide central apse is flanked
by two smaller side apses; thenceforward three apses became habitual,
demanded by the developed ritual; and at the west there is the usual
narthex. In Istanbul the clearest and grandest example of this type is
the church of St. Theodosia (Gül Camii), probably dating from the
eleventh century.
Another type, often considered as a mere development of the plan
of the cross-domed church, though it may have had an independent
origin, has been called by several names, but we shall identify it as
the four-column church because its most striking internal feature is
the four columns which here take the place of the corner piers of the
earlier type as supports for the dome. These churches are all small and
tall, more or less square on the exterior, but preserving the cruciform
plan within. There are no galleries, except sometimes over the narthex,
but the four corners of the cross are occupied by bays domed or with
domical vaults on high drums; these, together with the central dome,
form a quincunx, by which name this type is sometimes known. The
four-column church first appears in Istanbul in the ninth or tenth
century and thereafter became almost standard; its small size was
suitable to the declining revenues of the shrinking Empire, while its
interior form provided ample areas for mosaic and fresco decoration.
In Istanbul no less than eight examples survive, of which perhaps the