As early as the building of Constantine's churches in Palestine there were two chief types of plan in use: the basilican, or axial, type, represented by the basilica at the Holy Sepulchre, and the circular, or central, type, represented by the great octagonal church once at Antioch.

Those of the latter type we must suppose were nearly always vaulted, for a central dome would seem to furnish their very purpose.
The central space was sometimes surrounded by a very thick wall, in which deep recesses, to the interior, were formed, as at Church of St. George, Sofia, built by the Romans in the 4th century as a cylindrical domed structure built on a square base, and the noble Church of Saint George, Thessaloniki (5th century), or by a vaulted aisle, as at Santa Costanza, Rome (4th century); or annexes were thrown out from the central space in such a way as to form a cross, in which these additions helped to counterpoise the central vault, as at the Mausoleum of Galla Placidia, Ravenna (5th century).
The most famous church of this type was that of the Holy Apostles, Constantinople.
Vaults appear to have been early applied to the basilican type of plan; for instance, at Hagia Irene, Constantinople (6th century), the long body of the church is covered by two domes.

At Saint Sergius, Constantinople, and San Vitale, Ravenna, churches of the central type, the space under the dome was enlarged by having apsidal additions made to the octagon.
Finally, at Hagia Sophia (6th century) a combination was made which is perhaps the most remarkable piece of planning ever contrived.
A central space of 100 ft (30 m) square is increased to 200 ft (60 m) in length by adding two hemicycles to it to the east and the west; these are again extended by pushing out three minor apses eastward, and two others, one on either side of a straight extension, to the west.
This unbroken area, about 260 ft (80 m) long, the larger part of which is over 100 ft (30 m) wide, is entirely covered by a system of domical surfaces.
Above the conchs of the small apses rise the two great semi-domes which cover the hemicycles, and between these bursts out the vast dome over the central square.
On the two sides, to the north and south of the dome, it is supported by vaulted aisles in two stories which bring the exterior form to a general square.

At the Holy Apostles (6th century) five domes were applied to a cruciform plan; the central dome was the highest.
After the 6th century there were no churches built which in any way competed in scale with these great works of Justinian, and the plans more or less tended to approximate to one type.
The central area covered by the dome was included in a considerably larger square, of which the four divisions, to the east, west, north and south, were carried up higher in the vaulting and roof system than the four corners, forming in this way a sort of nave and transepts.
Sometimes the central space was square, sometimes octagonal, or at least there were eight piers supporting the dome instead of four, and the nave and transepts were narrower in proportion.

If we draw a square and divide each side into three so that the middle parts are greater than the others, and then divide the area into nine from these points, we approximate to the typical setting out of a plan of this time.
Now add three apses on the east side opening from the three divisions, and opposite to the west put a narrow entrance porch running right across the front.
Still in front put a square court.
The court is the atrium and usually has a fountain in the middle under a canopy resting on pillars.
The entrance porch is the narthex.
Directly under the center of the dome is the ambo, from which the Scriptures were proclaimed, and beneath the ambo at floor level was the place for the choir of singers.
Across the eastern side of the central square was a screen which divided off the bema, where the altar was situated, from the body of the church; this screen, bearing images, is the iconostasis.
The altar was protected by a canopy or ciborium resting on pillars.
Rows of rising
As early as the building of Constantine's churches in Palestine there were two chief types of plan in use: the basilican, or axial, type, represented by the basilica at the Holy Sepulchre, and the circular, or central, type, represented by the great octagonal church once at Antioch. 
Those of the latter type we must suppose were nearly always vaulted, for a central dome would seem to furnish their very purpose. The central space was sometimes surrounded by a very thick wall, in which deep recesses, to the interior, were formed, as at Church of St. George, Sofia, built by the Romans in the 4th century as a cylindrical domed structure built on a square base, and the noble Church of Saint George, Thessaloniki (5th century), or by a vaulted aisle, as at Santa Costanza, Rome (4th century); or annexes were thrown out from the central space in such a way as to form a cross, in which these additions helped to counterpoise the central vault, as at the Mausoleum of Galla Placidia, Ravenna (5th century). The most famous church of this type was that of the Holy Apostles, Constantinople. Vaults appear to have been early applied to the basilican type of plan; for instance, at Hagia Irene, Constantinople (6th century), the long body of the church is covered by two domes.
At Saint Sergius, Constantinople, and San Vitale, Ravenna, churches of the central type, the space under the dome was enlarged by having apsidal additions made to the octagon. Finally, at Hagia Sophia (6th century) a combination was made which is perhaps the most remarkable piece of planning ever contrived. A central space of 100 ft (30 m) square is increased to 200 ft (60 m) in length by adding two hemicycles to it to the east and the west; these are again extended by pushing out three minor apses eastward, and two others, one on either side of a straight extension, to the west. This unbroken area, about 260 ft (80 m) long, the larger part of which is over 100 ft (30 m) wide, is entirely covered by a system of domical surfaces. Above the conchs of the small apses rise the two great semi-domes which cover the hemicycles, and between these bursts out the vast dome over the central square. On the two sides, to the north and south of the dome, it is supported by vaulted aisles in two stories which bring the exterior form to a general square.
At the Holy Apostles (6th century) five domes were applied to a cruciform plan; the central dome was the highest. After the 6th century there were no churches built which in any way competed in scale with these great works of Justinian, and the plans more or less tended to approximate to one type. The central area covered by the dome was included in a considerably larger square, of which the four divisions, to the east, west, north and south, were carried up higher in the vaulting and roof system than the four corners, forming in this way a sort of nave and transepts. Sometimes the central space was square, sometimes octagonal, or at least there were eight piers supporting the dome instead of four, and the nave and transepts were narrower in proportion.
If we draw a square and divide each side into three so that the middle parts are greater than the others, and then divide the area into nine from these points, we approximate to the typical setting out of a plan of this time. Now add three apses on the east side opening from the three divisions, and opposite to the west put a narrow entrance porch running right across the front. Still in front put a square court. The court is the atrium and usually has a fountain in the middle under a canopy resting on pillars. The entrance porch is the narthex. Directly under the center of the dome is the ambo, from which the Scriptures were proclaimed, and beneath the ambo at floor level was the place for the choir of singers. Across the eastern side of the central square was a screen which divided off the bema, where the altar was situated, from the body of the church; this screen, bearing images, is the iconostasis. The altar was protected by a canopy or ciborium resting on pillars. Rows of rising 
Byzantine architecture