In churches of the 'four column' type the full triple division is common but with a change in purpose. A gallery in a church of this character is not possible, for the piers between which the gallery was placed have dwindled into single shafts. Hence the first string-course ceases to mark a gallery level and becomes the abacus level of the dome columns, as in the north and in the south churches of the Pantokrator. It is then carried round the building, and forms the impost moulding of the side arches in the bema and of the east window. Sometimes, however, it does not extend round the bema and apse but is confined to the central part of the church, as in the Myrelaion, S. Theodore, and the Pantepoptes. On the other hand, in at least one case, the parecclesion of the Pammakaristos, the central part of the chapel is designed in the usual three tiers, but the apse and bema vaults spring from the lower or abacus string-course, leaving a lunette in the dome arch above pierced by a large window. A corresponding lunette at the west end opens into the gynecaeum of the chapel. In S. John in Trullo the two string-courses coalesce and the arches connecting the columns with the walls cut into the stilted part of the dome arches, with the result that all the structural arches and vaults spring from the same level. [Illustration: FIG. 7. (Map of BYZANTINE CONSTANTINOPLE)] _Arches._--Though the pointed arch was known and employed in cisterns, as in the Cistern of the One Thousand and One Columns, Bin-bir-derek, the circular arch is invariably found in work meant to be seen. The difficulty attending this form, in which arches of unequal breadth do not rise to the same height, was overcome, as in the West, by stilting, that is, by raising the smaller arches on straight 'legs' to the required height. The stilted arch, indeed, seems to have been admired for its own sake, as we find it used almost universally both in vaulting and in decorative arches even where it was not structurally required. In windows and in the arches connecting the dome columns to the wall stilting is sometimes carried to extremes. _Domes._--The eastern dome of S. Irene, erected about 740 A.D., is generally considered to be the first example of a dome built on a high drum, though S. Sophia of Salonica, an earlier structure, has a low imperfect drum. After this date the characteristics of the Byzantine dome are the high drum divided by ribs or hollow segments on the interior, polygonal on the exterior, and crowned by a cornice which is arched over the windows.[26] Drumless domes are sometimes found in the later churches, as in the narthexes of the Panachrantos and S. Andrew, the angle domes of S. Theodosia, and in Bogdan Serai. These are ribless hemispherical domes of the type shown in Fig. 8, and are in all cases without windows. The earlier system of piercing windows through the dome does not occur in the later churches, though characteristic of Turkish work.