There is no absolute line of demarcation, chronological, geographical, or structural, between Early Christian and Byzantine architecture. But the former was especially characterized by the basilica with three or five aisles, and the use of wooden roofs even in its circular edifices; the vault and dome, though not unknown, being exceedingly rare. Byzantine architecture, on the other hand, rarely produced the simple three-aisled or five-aisled basilica, and nearly all its monuments were vaulted. The dome was especially frequent, and Byzantine architecture achieved its highest triumphs in the use of the _pendentive_, as the triangular spherical surfaces are called, by the aid of which a dome can be supported on the summits of four arches spanning the four sides of a square, as explained later. There is as little uniformity in the plans of Byzantine buildings as in the forms of the vaulting. A few types of church-plan, however, predominated locally in one or another centre; but the controlling feature of the style was the dome and the constructive system with which it was associated. The dome, it is true, had long been used by the Romans, but always on a circular plan, as in the Pantheon. It is also a fact that pendentives have been found in Syria and Asia Minor older than the oldest Byzantine examples. But the special feature characterizing the Byzantine dome on pendentives was its almost exclusive association with plans having piers and columns or aisles, with the dome as the central and dominant feature of the complex design (see plans, Figs. 74, 75, 78). Another strictly Byzantine practice was the piercing of the lower portion of the dome with windows forming a circle or crown, and the final development of this feature into a high drum. +CONSTRUCTION.+ Still another divergence from Roman methods was in the substitution of brick and stone masonry for concrete. Brick was used for the mass as well as the facing of walls and piers, and for the vaulting in many buildings mainly built of stone. Stone was used either alone or in combination with brick, the latter appearing in bands of four or five courses at intervals of three or four feet. In later work a regular alternation of the two materials, course for course, was not uncommon. In piers intended to support unusually heavy loads the stone was very carefully cut and fitted, and sometimes tied and clamped with iron.