[Illustration: Plate V. Arch Masonry. BRULETTO OF COMO.] § XXII. In the above reasonings, the inclination of the joints of the voussoirs to the curves of the arch has not been considered. It is a question of much nicety, and which I have not been able as yet fully to investigate: but the natural idea of the arrangement of these lines (which in round arches are of course perpendicular to the curve) would be that every voussoir should have the lengths of its outer and inner arched surface in the same proportion to each other. Either this actual law, or a close approximation to it, is assuredly enforced in the best Gothic buildings. § XXIII. I may sum up all that it is necessary for the reader to keep in mind of the general laws connected with this subject, by giving him an example of each of the two forms of the perfect Gothic arch, uncusped and cusped, treated with the most simple and magnificent masonry, and partly, in both cases, Mont-Cenisian. The first, Plate V., is a window from the Broletto of Como. It shows, in its filling, first, the single-pieced arch, carried on groups of four shafts, and a single slab of marble filling the space above, and pierced with a quatrefoil (Mont-Cenisian, this), while the mouldings above are each constructed with a separate system of voussoirs, all of them shaped, I think, on the principle above stated, § XXII., in alternate serpentine and marble; the outer arch being a noble example of the pure uncusped Gothic construction, _b_ of Plate III. [Illustration: Fig. XXXIV.]