Hagia Sophia is the supreme masterpiece of Byzantine architecture. Its spacious nave is covered by a lofty central dome carried on pendentives, a device not previously employed in monumental construction. Pendentives make possible support of the dome on a square framework of four huge equal arches resting on huge piers. The arches at the east and west are extended and buttressed by great half domes, while the half domes in turn are carried on smaller semidomed exedrae. A vast oblong interior, 102 ft (31 m) by 265 ft (81 m), is thus created from a succession of domical elements that build up to the main dome, 102 ft (31 m) in diameter and 184 ft (56 m) high, in which a corona of 40 arched windows sheds a flood of light on the interior.
At the east end of the nave is the vaulted sanctuary apse and at the west end a great narthex or vestibule, beyond which an exonarthex opens to the forecourt, or atrium. Flanking the nave to the north and south are side aisles with galleries over them. Their massive vaults, carried at both levels by monolithic columns of green and white marble and purple porphyry, serve as buttresses to receive the thrust of the great dome and its supporting arches. The vast interior is thus wholly free of suggestion of ponderous load, and its effect is that of a weightless golden shell that seems to possess a miraculous inherent stability.
In this one structural organism the Roman methods of construction are epitomized, modified and enriched by new aesthetic theories and realized in strikingly colorful materials and ornamental techniques. These materials and techniques are often considered Eastern, but they are in fact the logical outgrowth of trends already apparent in Roman imperial buildings of the first three centuries A.D. All interior surfaces are sheathed with polychrome marbles and gold mosaic, encrusted upon the brick core of the structure; most of the magnificent figure mosaics have been cleaned and restored to view. Externally, the broad, smooth surfaces of stuccoed walls and the great unconcealed masses of vaults and domes pile up impressively. Hagia Sophia served as model for several of the great Turkish mosques of Constantinople.