The doorways are often placed on the north and south sides and provided with porches. The windows, in the Early English Period, are either of simple lancet form, used singly or grouped in pairs or threes, united over the top by tracery; or the lights terminate at the top in trefoils or cinquefoils, a heading rare in France known as _cusps_. Plate tracery is comparatively rare in England, and the tracery of the Decorated Period is of a geometric character. Gradually the tracery becomes more flowing, including lines of double curvature, which somewhat resembles the French Flamboyant. Finally in the Perpendicular Period, the pointed arch of the window becomes flattened until it reaches the slight curve of the so-called four-centre arch. The window is divided up into many lights (nine in =King’s College Chapel=, =Cambridge=) by vertical mullions, which in the larger examples are reinforced by horizontal transoms, thus producing parallel rows of cusped lights, surmounted in the heading by small replicas of the latter in several tiers--an arrangement somewhat rigid and monotonous. The mouldings are generally richer and more varied than in French cathedrals, and, owing to their being nearer to the eye, are wrought to a greater refinement of finish. =Ornament.=--The characteristic ornament of the Early period is the “dog-tooth” used profusely in hollow mouldings. The foliage is conventionalised, crisply carved in bold curving and curling masses, known as “stiff-leaf foliage,” which in the case of capitals sometimes projects beyond the abacus. The flat surfaces of the walls are often diapered. In the Decorated Period the characteristic ornament of hollow mouldings is the “ball-flower”; but the “four-leaf flower” is frequently used. By degrees other motives of ornament were drawn from ivy, oak, and vine leaves; and the treatment grew more and more naturalistic. In the Tudor period the special ornament is the Tudor rose, two concentric layers of five petals, symbolising the union of the Houses of York and Lancaster. Other frequent motives are the portcullis and, in reference to the continued claim of the English kings to the throne of France, the fleur-de-lys. The traceries of the windows were repeated in the panelling of the walls and in the elaborate choir and chancel screens, which were a special feature of the period. =Stained Glass.=--Stained glass played a very important part in the embellishment of cathedrals and churches. York Minster presents an opportunity of studying the progress of the art through some four centuries, as it was practised, not only in England, but throughout Europe.