1988. The transparent covering, or glazed frame or sash, consists of a boundary frame, composed of two side pieces called styles, and two end pieces called the top and bottom rails, with the interspace divided by rebated bars to contain the glass. It is used as the opaque covering frames, and has the advantage of them in admitting abundance of light. In general, the rebated bars are inserted in one plane, as in common hotbed sashes; but, in some cases, the surface is in angular ridges, or ridge and furrow work (fig. 556.); cuneiform (fig. 557.); or trigonal (fig. 558.); in order, in each of these cases, to admit more of the rays of the sun in the morning and afternoon, and to moderate it in the middle hours of the day. This ridge and furrow surface may also be adopted where very flat roofs are to be glazed, as it will carry off the water better than any other; every ridge delivering the water to its furrow; and the accumulation there being such as to force its way off by its own gravity. Such frames are used for placing over beds of hot dung, for growing cucumbers, forcing roots or flowers, and for a great variety of purposes. The materials of sashes are commonly timber; but iron, cast and wrought, and copper are also used.