The wall of a building ordinarily requires some kind of base and some kind of top. The base or plinth in English Gothic buildings was usually well marked and bold, especially in the perpendicular period, and it is seldom absent. The eaves of the roof in some cases overhang the walls, resting on a simple stone band, called an eaves-course, and constitute the crowning feature. In many instances, however, the eaves are concealed behind a parapet[10] which is often carried on a moulded cornice or on corbels. This, in the E. E. period, was usually very simple. In the Dec. it was panelled with ornamental panels, and often made very beautiful. In the Perp. it was frequently battlemented as well as panelled. A distinguishing feature of Gothic walls is the buttress. It existed, but only in the form of a flat pier of very slight projection in Norman, as in almost all Romanesque buildings, but in the Gothic period it became developed. The buttress, like many of the peculiarities of Gothic architecture, originated in the use of stone vaults and the need for strong piers at these points, upon which the thrust and weight of those vaults were concentrated. The use of very large openings, for wide windows full of stained glass also made it increasingly necessary in the Dec. and Perp. periods to fortify the walls at regular points. A buttress[10] is, in fact, a piece of wall set athwart the main wall, usually projecting considerably at the base and diminished by successive reductions of its mass as it approaches the top, and so placed as to counteract the thrust of some arch or vault inside. It had great artistic value; in the feeble and level light of our Northern climate it casts bold shadows and catches bright lights, and so adds greatly to the architectural effect of the exterior. In the E. E. the buttress was simple and ordinarily projected about its own width. In the Dec. it obtained much more projection, was constructed with several diminutions (technically called weatherings), and was considerably ornamented. In the Perp. it was frequently enriched by panelling. The buttresses in the Dec. period are often set diagonally at the corner of a building or tower. In the E. E. period this was never done. The flying buttress[11] is one of the most conspicuous features of the exterior of those Gothic buildings which possessed elaborate stone vaults. It was a contrivance for providing an abutment to counterbalance the outward pressure of the vault covering the highest and central parts of the building in cases where that vault rested upon and abutted against walls which themselves were carried by arches, and were virtually internal walls, so that no buttress could be carried up from the ground to steady them.