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. LIGHTING PROBLEMS CONNECTED WITH THE CONSTRUCTION OF HALF DOMED APSES It is a characteristic of the half dome that it exerts a large amount of downward pressure and but little outward thrust, particularly if it be of pointed section. For this reason, such a vault requires a firm support but only a slight amount of buttressing. As long, therefore, as the half dome rested directly upon comparatively low exterior walls, it had plenty of support, and it was even possible to pierce the walls with windows without endangering its stability. But with the increase in height of the more developed Romanesque churches and the introduction of ambulatories, it became difficult to light the sanctuary and still retain the half dome. Two methods were evolved for overcoming this difficulty. The first, which may be seen in the abbey church at Cunault (Maine-et-Loire) (second half of twelfth century),[339] consisted in the construction of a lofty ambulatory opening into the apse through arches rising to the impost of the half dome, or even slightly above it, and resting upon piers of as slender proportions as possible, so that, although the sanctuary was deprived of all direct light, a certain amount was obtained from windows in the outer wall of the ambulatory or from the radiating chapels, while, at the same time, the vault of this ambulatory aided in the support of the apse and vice versa. Such a system, though structurally correct, was not entirely satisfactory. The sanctuary and choir were the portions of the church most in need of lighting, since they contained the altar and the seats for the monks or clergy by whom the services were chanted, and indirect light was bound to be insufficient.