_Building Procedure._--The first step in the erection of a building was to obtain the necessary marble columns with their capitals and bases. These seem to have been largely supplied ready made, and Constantinople was a great centre for the manufacture and export of stock architectural features. Then the main walls were built in brick, the columns were inserted as required, the vaults were thrown, and the whole building was left to settle down. Owing to the enormous amount of mortar used this settling must have been very considerable, and explains why hardly a plumb wall exists in Constantinople, and why so many vaults show a pronounced sinking in at the crown or have fallen in and have been rebuilt. After the walls had set the marble facings, mosaic, and colour were applied and could be easily adapted to the irregular lines of the walls. Byzantine architecture made little use of mouldings. The great extension of flat and spacious decoration rendered unnecessary, or even objectionable, any strong line composition. External cornices are in coursed brick, the alternate courses being laid diagonally so as to form the characteristic dentil. The richest form is that found in the Pammakaristos, S. Theodosia, and S. Thekla, where the small dentil cornice is supported on long tapering corbels, a design suggested by military machicolations. [Illustration: PLATE IV. (1) S. SAVIOUR IN THE CHORA. BRACKET IN THE INNER NARTHEX. (2) S. THEODORE. SCULPTURED MARBLE SLAB BUILT INTO THE MINARET OF THE MOSQUE. (3) S. MARY DIACONISSA. HEADS OF WINDOWS IN SOUTH ARM. (4) S. MARY DIACONISSA. SCULPTURED SLAB ON THE WEST WALL. _To face page 28._] The stone ogee, cavetto, or cavetto and bead cornice is common, but seems in every case to be Turkish work and is very common in Turkish buildings. Internal cornices and string-courses are in marble, and are all of the same type, a splay and fillet. The splayed face is decorated with upright leaves or with a guilloche band, either carved (in the Pantepoptes) or painted (in the Chora), the carving as in classic work, serving only to emphasise the colour. The splay is sometimes slightly hollowed, sometimes, as in the Chora, worked to an ogee.