Another result of the framework system of structure was that the intervening wall-spaces, relieved of strain, could be fully utilised for openings, especially for windows, so important in the duller climate of the north. The clerestory became an important feature of the Gothic cathedral; so also the _triforium_, or gallery round the nave, which, pierced in the thickness of the wall, separated the clerestory and arcade arches. Further, the windows in all the outer walls took on a new importance. =Windows.=--The windows, in fact, became another of the distinguishing characteristics of Gothic architecture and the variety in their treatment marks the several centuries of its development. At first there was the plain _lancet_ (spear-headed) window, the top of which was composed of two segments of a circle meeting at one point. The segments were inscribed about a triangle, which was either equilateral or isosceles. In the case of the equilateral triangle, whose base was equal to the sides, the distance of the point of the arch from the spring of the curves was equal to the width of the window. On the other hand, in the case of the isosceles triangle, if the base were longer than the sides, the point of the arch dropped lower, while, if the base were shorter, the arch was higher than its width--the true lancet. Such plain openings, or _lights_, were used either singly or in pairs; and in time two were included within one lancet opening, the space above the heads of the lights being filled with a round or _quatrefoil_ light. In this case the upper part or _tracery_ had the appearance of having been cut out of one slab or plate of stone, and the pattern in consequence was called _plate-tracery_. Later, when the number of lights in a window was increased, the tracery above them was elaborated into various geometric designs, technically known as _bar-tracery_. Still later, when the architects had completely solved all the structural problems and the only advance could be made in further elaboration of details, the geometric forms were abandoned for more flowing designs, which are called in French Gothic _Flamboyant_; in English, _Decorated_. It is to be noted that the change in the treatment of the windows was reflected in the carved ornamental details of other parts of the edifice; especially in the canopies over niches and the embellishment of gables, doors, choir-screens, wall-panelling, finials, and spires. These in the Flamboyant period (fifteenth century) reached a degree of lace-like elaborateness, that, while beautiful in itself, tends to obscure the actual structural elements; thereby marking the decadence of the Gothic style. This phase was represented in English Gothic by a gradual stiffening of the tracery into rigid forms and barren repetitions. Because of the insistence on rectangular motives it is known as _Perpendicular_.