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.

The three diagrams (Figs. 8, 9, and 10) illustrate the development of
the dome: firstly, the low saucer dome or dome-vault in which dome and
pendentives are part of the same spherical surface; secondly, the
hemisphe