Apses Vaulted with Half Domes
Once adopted from Roman architecture as a standard part of the church plan, the construction of the apse was naturally based upon Roman models, and since these were always vaulted with a half dome of masonry, similar half domes were employed by the Christian builders of the early mediaeval period. During the Romanesque era, these half domes were almost always of stone laid in horizontal courses, supported by substantial walls of semicircular or polygonal plan. They opened directly into a transept or a tunnel-vaulted choir. The earliest of these half domes were of semicircular elevation, but the pointed form made its appearance in the late eleventh or early twelfth century in many churches. In both forms, the principles of construction are the same.