+THE POINTED ARCH+ was adopted to remedy the difficulties encountered in the construction of oblong vaults. It is obvious that where a narrow semi-cylindrical vault intersects a wide one, it produces either what are called _penetrations_, as at a (Fig. 109), or intersections like that at b, both of which are awkward in aspect and hard to construct. If, however, one or both vaults be given a pointed section, the narrow vault may be made as high as the wide one. It is then possible, with but little warping of the vaulting surfaces, to make them intersect in groins c, which are vertical plane curves instead of wavy loops like a and b. The Gothic architects availed themselves to the full of these two devices. They built their groin-ribs of semi-circular or pointed form, but the wall-ribs and the transverse ribs were, without exception, pointed arches of such curvature as would bring the apex of each nearly or quite to the level of the groin intersection. The pointed arch, thus introduced as the most convenient form for the vaulting-ribs, was soon applied to other parts of the structure. This was a necessity with the windows and pier-arches, which would not otherwise fit well the wall-spaces under the wall-ribs of the nave and aisle vaulting. +TRACERY AND GLASS.+ With the growth in the size of the windows and the progressive suppression of the lateral walls of vaulted structures, stained glass came more and more generally into use. Its introduction not only resulted in a notable heightening and enriching of the colors and scheme of the interior decoration, but reacted on the architecture, intensifying the very causes which led to its introduction. It stimulated the increase in the size of windows, and the suppression of the walls, and contributed greatly to the development of _tracery_. This latter feature was an absolute necessity for the support of the glass. Its evolution can be traced (Figs, 110, 111, 112) from the simple coupling of twin windows under a single hood-mould, or discharging arch, to the florid net-work of the fifteenth century. In its earlier forms it consisted merely of decorative openings, circles, and quatrefoils, pierced through slabs of stone (_plate-tracery_), filling the window-heads over coupled windows. Later attention was bestowed upon the form of the stonework, which was made lighter and richly moulded (_bar-tracery_), rather than upon that of the openings (Fig. 111). Then the circular and geometric patterns employed were abandoned for more flowing and capricious designs (_Flamboyant_ tracery, Fig. 112) or (in England) for more rigid and rectangular arrangements (_Perpendicular_, Fig. 134). It will be shown later that the periods and styles of Gothic architecture are more easily identified by the tracery than by any other feature. [Illustration: FIG. 110.--PLATE TRACERY, CHARLTON-ON-OXMORE.]