This is the form of all good doors, without exception, over the whole world and in all ages, and no other can ever be invented. § III. In the simplest doors the cross lintel is of wood only, and glass or bars occupy the space above, a very frequent form in Venice. In more elaborate doors the cross lintel is of stone, and the filling sometimes of brick, sometimes of stone, very often a grand single stone being used to close the entire space: the space thus filled is called the Tympanum. In large doors the cross lintel is too long to bear the great incumbent weight of this stone filling without support; it is, therefore, carried by a pier in the centre; and two valves are used, fitted to the rectangular spaces on each side of the pier. In the most elaborate examples of this condition, each of these secondary doorways has an arch heading, a cross lintel, and a triangular filling or tympanum of its own, all subordinated to the main arch above. § IV. 2. Fillings of windows. When windows are large, and to be filled with glass, the sheet of glass, however constructed, whether of large panes or small fragments, requires the support of bars of some kind, either of wood, metal, or stone. Wood is inapplicable on a large scale, owing to its destructibility; very fit for door-valves, which can be easily refitted, and in which weight would be an inconvenience, but very unfit for window-bars, which, if they decayed, might let the whole window be blown in before their decay was observed, and in which weight would be an advantage, as offering more resistance to the wind. Iron is, however, fit for window-bars, and there seems no constructive reason why we should not have iron traceries, as well as iron pillars, iron churches, and iron steeples. But I have, in the "Seven Lamps," given reasons for not considering such structures as architecture at all. The window-bars must, therefore, be of stone, and of stone only. § V. The purpose of the window being always to let in as much light, and command as much view, as possible, these bars of stone are to be made as slender and as few as they can be, consistently with their due strength. [Illustration: Fig. XLV.] Let it be required to support the breadth of glass, _a_, _b_, Fig. XLV. The tendency of the glass sustaining any force, as of wind from without, is to bend into an arch inwards, in the dotted line, and break in the centre. It is to be supported, therefore, by the bar put in its centre, _c_. But this central bar, _c_, may not be enough, and the spaces _a c_, _c b_, may still need support. The next step will be to put two bars instead of one, and divide the window into three spaces as at _d_.