Flying buttresses probably of the ninth century are used at the west end of S. Sophia. The double-flying buttress to the apse of the Chora does not bond with the building and is certainly not original. It may be set down as part of the Byzantine restoration of the church in the fourteenth century. In any case, such external flying abutments are alien to the spirit of Byzantine architecture, and may be regarded as an importation from the West. Flying buttresses, it may here be noted, are not uncommon in the great mosques of the city. They are found in Sultan Bayazid, Rustem Pasha, Sultan Selim, the Suleimanieh, and the Shahzadé. But they are generally trifling in size, and are rather ornaments than serious attempts to buttress the dome. _Walls._--The walls of the earlier churches are built of large thin bricks laid with mortar joints at least as thick as the bricks, and often of greater thickness. Stone is used only in special cases, as in the main piers of S. Sophia, but monolithic marble columns are an important part of the structure. In the later churches stone is used in courses with the bricks to give a banded effect, and herring-bone, diamond, and radiating patterns are frequently introduced. The palace of the Porphyrogenitus, the parecclesion of the Pammakaristos, and Bogdan Serai, exhibit this style of work. As illustrations of the method adopted in the construction of walls the following measurements may be given, the sizes being in centimetres: +---------------------------------------+----------+--------------+ | | Brick. | Joint. | +---------------------------------------+----------+--------------+ | Parecclesion of the Pammakaristos | .08 | .04 | | 4 courses brick, 5 joints | .46 | --- | | S. John in Trullo | .03 | .07 to .09 | | Refectory of the Monastery of Manuel | .04 | .04 to .06 | | 4 course stone, 3 joints | .78 | --- | | 4 courses brick, 5 joints | .30 | --- | | | { .0375 | .052 | | Bogdan Serai | { .035 | .035 | | | { .04 | .04 | | 4 courses stone, 8 joints | --- | .55 to .60 | | 4 courses brick, 5 joints | --- | .43 to .47 | | Sanjakdar, brick | .045 | --- | +---------------------------------------+----------+--------------+