The nave of St. Sernin, as will be observed (Woodcut No. 573), has double side-aisles, above the inner one of which runs a grand gallery. The roof of this gallery—in section the quadrant of a circle—forms an abutment to the roof of the nave, which is a bold tunnel-vault ornamented by transverse ribs only. So far the constructive arrangements are the same as in the transitional church of Fontifroide. Passing from the nave to the choir, both at Toulouse and at Conques, we come upon a more extended and complicated arrangement than we have hitherto met with. It will be recollected that the early Romanesque apse was a simple large niche, or semi-dome; so we found out in the Lombard style, and shall find it in the German style when it comes to be described, and generally even in the neighbouring Provençal style, and always—when unaltered—in the domical style last described. In the present instance it will be seen that a semicircular range of columns is substituted for the wall of the apse, an aisle bent round them, and beyond the aisle there are always three, five, or even seven chapels opening into it, which give it a complexity very different from the simple apse of the Roman basilicas and the other styles we have been describing, and at the same time a perspective and a play of light and shade which are unrivalled in any similar invention of the Middle Ages. The _apse_, properly speaking, is a solid semi-cylinder, surmounted by a semi-dome, but always solid below, though generally broken by windows above. The _chevet_, on the contrary, is an apse, always enclosed by an open screen of columns on the ground-floor, and opening into an aisle, which again always opens into three or more apsidal chapels. This arrangement is so peculiarly French, that it may properly be characterised by the above French word, a name once commonly applied to it, though latterly it has given way to the more classical, but certainly less suitable, term of apse. Its origin too is worth inquiring into, and seems to be capable of easy explanation. [Illustration: 574. Plan of Church at Conques. (From Taylor and Nodier.) Scale 100 ft. to 1 in.] The uses which the various nations of Christendom made of the circular form of building left them by the Romans have been more than once adverted to in this work. The Italians used it almost always standing alone as a tomb-house or as a baptistery; the Germans converted it into a western apse, while sometimes, as at Bonn and elsewhere, they timidly added a porch or nave to it; but the far more frequent practice with the Germans, and also in England, was to build first the circular church for its own sake, as in Italy: then the clergy for their own accommodation added a choir, that they might pray apart from the people.