§ II. In speaking of the arch-line and arch masonry, we considered the arch to be cut straight through the wall; so that, if half built, it would have the appearance at _a_, Fig. LXIX. But in the chapter on Form of Apertures, we found that the side of the arch, or jamb of the aperture, might often require to be bevelled, so as to give the section _b_, Fig. LXIX. It is easily conceivable that when two ranges of voussoirs were used, one over another, it would be easier to leave those beneath, of a smaller diameter, than to bevel them to accurate junction with those outside. Whether influenced by this facility, or by decorative instinct, the early northern builders often substitute for the bevel the third condition, _c_, of Fig. LXIX.; so that, of the three forms in that figure, _a_ belongs principally to the south, _c_ to the north, and _b_ indifferently to both. [Illustration: Fig. LXIX.] § III. If the arch in the northern building be very deep, its depth will probably be attained by a succession of steps, like that in _c_; and the richest results of northern archivolt decoration are entirely based on the aggregation of the ornament of these several steps; while those of the south are only the complete finish and perfection of the ornament of one. In this ornament of the single arch, the points for general note are very few. § IV. It was, in the first instance, derived from the classical architrave,[91] and the early Romanesque arches are nothing but such an architrave, bent round. The horizontal lines of the latter become semicircular, but their importance and value remain exactly the same; their continuity is preserved across all the voussoirs, and the joints and functions of the latter are studiously concealed. As the builders get accustomed to the arch, and love it better, they cease to be ashamed of its structure: the voussoirs begin to show themselves confidently, and fight for precedence with the architrave lines; and there is an entanglement of the two structures, in consequence, like the circular and radiating lines of a cobweb, until at last the architrave lines get worsted, and driven away outside of the voussoirs; being permitted to stay at all only on condition of their dressing themselves in mediæval costume, as in the plate opposite.