The building comprises two churches, which, while differing in date and type, stand side by side, and communicate with each other through an archway in their common wall, and through a passage in the common wall of their narthexes. As if to keep the two churches more closely together, they are bound by an exonarthex, which, after running along their western front, returns eastwards along the southern wall of the south church as a closed cloister or gallery. _The North Church._--The north church is of the normal 'four column' type. The four columns which originally supported the dome were, however, removed when the building was converted into a mosque in Turkish times, and have been replaced by two large pointed arches which span the entire length of the church. But the old wall arches of the dome-columns are still visible as arched piercings in the spandrils of the Turkish arches. A similar Turkish 'improvement' in the substitution of an arch for the original pair of columns is found in the north side of the parecclesion attached to the Pammakaristos (p. 152). The dome with its eight windows is likewise Turkish. The windows are lintelled and the cornice is of the typical Turkish form. The bema is almost square and is covered by a barrel vault formed by a prolongation of the eastern dome arch; the apse is lighted by a lofty triple window. By what is an exceptional arrangement, the lateral chapels are as lofty both on the interior and on the exterior as is the central apse, but they are entered by low doors. In the normal arrangement, as, for instance, in the Myrelaion, the lateral chapels are low and are entered by vaults rising to the same height as those of the angle chambers, between which the central apse rises higher both externally and internally. The chapels have niches arched above the cornice on three sides, and are covered by cross-groined vaults which combine with the semicircular heads of the niches to produce a very beautiful effect. To the east they have long bema arches flanked by two small semicircular niches, and are lighted by small single windows. The church is preceded by a narthex in three bays covered by cross-groined vaults supported on strong transverse arches. At either end it terminates in a large semicircular niche. The northern one is intact, but of the southern niche only the arched head remains. The lower part of the niche has been cut away to afford access to the narthex of the south church. This would suggest that, at least, the narthex of the south church is of later date than the north church. Considered as a whole the north church is a good example of its type, lofty and delicate in its proportions.