_The Openings_ are almost invariably arched, and seldom, if ever, covered by a lintel. It is hardly necessary to add that the arches are always round. Almost always they are semicircular, but instances of the employment of a segmental arch, or of one the outline of which is a little more than half a circle, may be occasionally met with. Door openings are often made important both by size and decoration. Window openings are usually small; and the grouping of two or more lights under one head, which was so conspicuous a feature in Gothic architecture, first appears in Byzantine buildings, and is met with also in Romanesque ones. The mode of introducing light is to a certain extent characteristic. The basilican churches always possess a clerestory, and usually side windows in the aisles; and this arrangement is generally followed in Romanesque buildings, though sometimes, in Germany, the clerestory is omitted. The gable ends of the nave and transepts are not usually pierced by many or large lights (Fig. 180); and when there is a central feature, as a tower, or even a dome, little or no light is introduced through it. On the other hand, the Byzantine churches depend largely for light upon the ring of windows which commonly encircles the base of the central dome, and sometimes that of the subsidiary domes; and the gables are pierced so as to supply any additional light required, so that windows are infrequent in the lower walls. Broadly speaking, therefore, the Western churches have side-lighting and the Eastern top-lighting. [Illustration: FIG. 180.--CHURCH AT ROSHEIM. UPPER PORTION OF FAÇADE.] The great arches which carry the main domes form a notable feature in Eastern churches, and are of very bold construction. In the basilican churches one great arch, called "the arch of triumph," occurs, and only one; this gives access to the apse: and a similar arch, which we now denominate "the chancel arch," usually occupies a corresponding position in all Romanesque churches. The arches of the arcade separating the nave from the aisles in all Western churches are usually of moderate span. In some ancient basilicas these arches are replaced by a horizontal beam.