The causes which led to the development of this style of window were probably two: one, the desire for more light, of which the richly coloured windows admitted but little; and the other, simply economy, for a window of this sort could be produced comparatively cheaply. Then, too, the Cistercians, whose rule, adopted in the twelfth century, prohibited the use of colour altogether, had shown what could be done in patterns of white glass and lead alone. Unless you count the "gryphon windows" in St. Denis, which are mainly the work of Viollet-le-Duc, grisaille seems almost wholly a development of the thirteenth century. It is interesting to see that just as the design of the coloured window seems always to have been conceived as a light pattern on a dark ground, so the earliest grisaille, even though the quantity of white far exceeds the colour, still seems to have been conceived as a white pattern on a coloured ground, the ground being, as it were, almost entirely hidden by the pattern. Later this idea gets reversed, and the coloured pieces are mere jewels or lines contained in the pattern. In a white window the leads, from their greater thickness, are more conspicuous than the traced lines of the painting, and in consequence it is upon the leads that the artist depends for the main features of his design. The earliest grisaille windows may be divided into two classes: those in which the pattern is formed of narrow "straps" of white glass interlacing or seeming to interlace; and the other in which the leads form a flat geometrical pattern, as at Lincoln. The painted pattern on the glass consists of branching scroll work in simple outline, forming stems and the round-lobed leaves which were the thirteenth century convention for foliage. In the earlier work the ground is covered with delicate cross-hatching, which at a distance resolves itself into a pearly grey, against which the scroll work stands out white. At first, too, the painted pattern is, so to speak, contained within the leading, and merely enriches and emphasizes the pattern formed by it; but in later work, towards the end of the century, it becomes independent of the leading and grows through it, spreading over the surface of the window in graceful curves like a creeper over a trellis. The influence of the medallion window is often seen in contemporary grisaille, of which the design frequently consists of interlacing medallions of strap-work of the same shape as those in the coloured windows. [Sidenote: Rheims.] [Sidenote: Angers, Soissons, and Chartres.]