_Apses._--A fully developed Byzantine church terminated in three apses: a large apse, with the bema or presbytery, in the centre; on the right, the apse of the prothesis where the sacrament was prepared; on the left, the apse of the diaconicon, where the sacred vessels were kept. Although there is proof that the prothesis and the diaconicon were in use at a very early period, yet many churches of the great period, as for example S. John of the Studion, SS. Sergius and Bacchus, and S. Sophia, dispensed with these chambers as distinct parts of the building. They were also omitted in small churches of a late date, where they were replaced by niches on either side of the bema. The three apses usually project from the east wall of the church, but occasionally (p. 248) the two lateral apses are sunk in the wall, and only the central apse shows on the exterior. As a rule the apses are circular within and polygonal without. It is rare to find them circular on both the interior and the exterior (p. 203), and in Greece such a feature is generally an indication of late date. An octagonal plan, in which three sides of the octagon appear, sometimes with short returns to the wall, is the most common; but in later churches polygons of more sides are used, especially for the central apse, and these are often very irregularly set out. Some of the churches of Constantinople show five, and even seven sides. _Bema._--The bema is rectangular, and sometimes has concave niches on each side (p. 130). It is covered either with a barrel or with a cross-groined vault, and communicates with the prothesis and the diaconicon. _Prothesis and Diaconicon._--These chambers are either square (p. 214) or have a long limb to the east resembling a miniature bema (p. 214). They are lower than the central apse and the cross arms, so that the cruciform figure of the church shows clearly above them on the exterior,[25] though in some churches with galleries small chapels overlooking the bema are placed above them at the gallery level (S. Theodosia). They have usually a niche on three sides, and are either dome vaulted or have cross-groined vaults. The combination of a cross-groined vault with four niches springing from the vaulting level is particularly effective. In S. Saviour in the Chora (p. 307) these chambers are covered with drum domes, pierced with windows, but this treatment is quite exceptional.