The new style would however, appear to have made rapid progress in the two centuries which followed, for in no other way would it be possible to account for the magnificence both structurally and artistically of the church (now the mosque) of St. Sophia at Constantinople, which was erected by the Emperor Justinian (commenced 528 A.D.), and which not only marks the culminating period of Byzantine architecture, but is still one of the great masterpieces of the art. An earlier building, ascribed also to Justinian art, which is said to have been built on the foundation of an earlier church by Constantine, viz. the church of St. Sergius and Bacchus (known as the lesser St. Sophia) indicates the direction in which the Byzantine architects were tending. The defect of the ordinary basilica lay in its timber roof, so easily destroyed by fire. Already in the basilica of Maxentius at Rome, completed by Constantine, and the remains of which still exist, a vault of prodigious space, 80 feet, had been thrown across the nave, and there is no doubt that this would have been the type selected by Constantine if, in the foundation of his new city, he could have undertaken so great a work; in fact, in his letters to Macarius, Bishop of Jerusalem, transcribed in Eusebius, he suggests the covering of his church by some other material than that of wood. It was left, however, for Justinian to realise the dream, and in the church of St. Sophia to produce a structure homogeneous in its material throughout, and covered with a magnificent vault.
The church of St. Sergius and Bacchus, already referred to, is octagonal in plan, and covered with a dome which is carried on arches supported by eight piers. The problem which Justinian attempted to solve was to support a dome on arches carried by four piers. The plan of the four arches being square, whereas the dome is circular on plan, it became necessary to build on the extrados of the arches what are known as pendentives, spherical triangles to fill the space between, and support the base of the dome. As the dome was 107 feet in diameter, those spherical triangles are about 70 feet wide at the top and 52 feet high, being, therefore, of colossal size. The means adopted to build these pendentives is not known, and two failures which happened in the great arches are described by Procopius, an historian of the period. Only twenty years after the erection (558 A.D.) a portion of the dome was overthrown by an earthquake, and a new dome, with forty circular-headed windows at its base, was erected in its place, the actual effect being, as described by Procopius, "as if it was sustained by a chain from heaven." The two side arches, north and south, were filled with a wall pierced with windows and arcades on two storeys, and immense apses were thrown out towards the east and west ends, so that the plan is that of an oblong square. The lower portions of the walls are panelled with marble, in which material are also the arcades with their columns and capitals; the remainder of the interior is covered with mosaics, which, as they represent figure subjects, forbidden by the Mohammedan religion, are now covered with stucco and painted. The exterior, owing to the flatness of the dome and the solidity and size of the buttresses and masonry round, does not convey any idea of the beauty of the interior. The type of church thus conceived and carried out by Justinian became the example on which has been based the greater number of churches devoted to the Greek ritual not only in Greece but throughout Russia. No attempt, however, has since been made to produce a dome of such great size, and the: subsequent examples have rarely exceeded 50 feet, in diameter. In order to give increased space, however, the nave and choir were lengthened, and transepts were thrown out on each side of the central dome, and these were also covered by domes, the best example of which is that found in St. Mark's at Venice (the present external domes of this church are only of timber covered with lead and do not belong to the original structure). The principal difference to be noted in the later Greek churches was the raising of the dome on cylindrical walls of masonry or brickwork pierced with windows. Of the fifth and sixth centuries there still exist at Thessalonica and elsewhere churches of the ordinary basilica type with timber roofs, which differ from the Roman examples chiefly in having arches instead of architraves to carry the nave walls. The influence of Byzantine architecture on Western architecture besides St. Mark's is seen in the churches of St. Vitale, St. Apollinare, in Navem, and St. Apollinarer-in-classe, all in Ravenna, and in the south of Italy and Sicily. At Monreale near Palermo is a magnificent basilica church with marble panelling and mosaic decoration to the internal walls. Many of the earlier Romanesque churches of Rome have the vaults of their apses covered with Byzantine mosaics, and in the south of France at St. Front-de-Perigueux, and in the Charente we find the dome as a characteristic feature, owing, probably, to the settlement of Greek artists in the south of France.