The courses form the walls. They are arranged in horizontal bands, with vertical and sometimes sloping joints. The separate stones are often bound together upon their horizontal surfaces by dovetails or tenons of wood. The blocks made use of in this form of construction are usually of large dimensions, but the Egyptians also made use of small stones or rubble, lined on the exterior by large flat ones which concealed the meanness of the material behind them.[103] (Fig. 70.) [103] We find this construction in the so-called Temple of the Sphinx, near the Great Pyramid. Various peculiarities of construction which are comparatively seldom met with will be noticed when we come to describe the monuments in which they are to be found. Architraves were stone beams used to bridge over the voids and to support the covering of the building, which latter was composed of long and heavy slabs. The vertical elements support the architraves and combine them one with another. These vertical supports vary greatly in size. Those of small or medium dimensions are monoliths; others are composed of many courses of stone one upon another, courses which in this case take the name of _drums_. [Illustration: FIG. 69.--The Egyptian "bond."] Upon exterior surfaces, supports take various forms of development which may all be referred to the type which we have defined, namely, the portico. In the interiors the form of support is a logical consequence of the material employed. Whenever the stones which form the roof are too small to bridge over the whole of the space comprised within two walls, they must be made to rest upon intermediate supports; and this necessity springs up in every building of any importance. This very elementary combination fulfils all the requirements of circulation. The number of supports depends upon the number of rows of the flat stones which form the roof. They are sometimes multiplied to such an extent that they remind us of that planting arrangement in our gardens which we call a quincunx. [Illustration: FIG. 70.--Double-faced wall.] [Illustration: FIGS. 71, 72.--Elements of the portico.] We cannot, however, affirm that the number of supports is invariably decided by the length of the architraves, or of the roofing stones. Some very long monoliths are supported at regular intervals, lest they should break with their own weight or with that put upon them. The walls, architraves, and vertical supports are always far stronger than the mere weight of the roof would require. The following woodcut shows the arrangement of supports, architraves and roof. These simple arrangements constitute a complete system of construction which, belonging exclusively to Egypt, has had results upon which we cannot too strongly insist. Both roof and architraves being horizontal, all the pressure upon the walls is vertical. There is no force tending to thrust the walls outwards nor to affect the immobility of the supports.