§ 1. We have hitherto considered the aperture as merely pierced in the thickness of the walls; and when its masonry is simple, and the fillings of the aperture are unimportant, it may well remain so. But when the fillings are delicate and of value, as in the case of coloured glass, finely wrought tracery, or sculpture, such as we shall often find occupying the tympanum of doorways, some protection becomes necessary against the run of the rain down the walls, and back by the bevel of the aperture to the joints or surface of the fillings.

§ 2. The first and simplest mode of obtaining this is by channelling the jambs and arch head; and this is the chief practical service of aperture mouldings, which are otherwise entirely decorative. But as this very decorative character renders them unfit to be made channels for rain water, it is well to add some external roofing to the aperture, which may protect it from the run of all the rain, except that which necessarily beats into its own area. This protection, in its most usual form, is a mere dripstone moulding carried over or round the head of the aperture.