Once the weight of the vault had been transferred to piers, the wall connecting them ceased to support anything but the extremity of the cross-vault comprised between the piers, and otherwise served only as a screen. The Gothic architects soon took advantage of this to widen the windows, which had been narrow in the early stages, for by throwing a discharging arch just under the upper vault across the piers the whole space underneath could be occupied by windows, which, with the improvement in the making of painted glass, became extremely desirable. This was accordingly done, the only stonework left being the network of mullions and tracery necessary to receive the panes. This tracery, probably suggested by the rich Arabic window fillings, made a great advance during the latter part of the thirteenth and fourteenth centuries, the combinations of geometrical figures, chiefly the circle, being often wonderfully beautiful. The rose window was much favoured by the French in their West fronts and transepts, but in England the large pointed window was generally preferred, and admirably suited the square termination of the apse, which was the most frequently used in that country. The space enclosed by the pointed window had an outline to which it was always difficult to adjust geometric traceries so as to avoid clumsy joints, or oddly shaped patterns, and these were, therefore, subsequently replaced by flowing lines, which could be used with much greater freedom. As these grew bolder they assumed a flame-like appearance, and the later period of the style to which they belong was, in consequence, called “Flamboyant.” This development occurred chiefly in France, some of the best examples being in the church of St. Ouen, at Rouen. The simplest form of the Gothic vault was that in which the compartment comprised between two piers on one side and two on the opposite side of the nave was marked by two ribs bridging it, and two diagonal ribs intersecting each other. As the system advanced the vault became more complex by the addition of other ribs, as strengtheners or as ornaments, until in some examples the whole vault became a network of intersecting ribs. These intersections were frequently emphasized by a keystone or by an ornament called a boss, which in English work was also placed at intervals along string-courses, breaking the continuity after the manner of modillions in Classic cornices.