(_β_) That which columns support is the _entablature_ superimposed. The relation of most importance to be considered here is that of _rectangularity._ Not merely in its relation to the ground, but also in that to the entablature the supporting structure must be rectangular. For the horizontal position is by the laws of gravity that which is alone intrinsically the most stable and fitting, and the right angle the only definitely secure one. The acute and obtuse angles are, on the contrary, indefinite, and both vary in their degree and are subject to contingency. We may differentiate between the component parts of the entablature as follows: (_αα_) The _architrave_, that is, the main beam, rests immediately upon the columns which stand adjacent in a direct line of equal height; this unites the columns together and places on them a weight shared equally. As beam, and nothing more, it merely requires the form of four level surfaces mutually related as rectangular in all three dimensions and their abstract regularity. Owing to the fact, however, that the architrave as to one part of it is supported by the columns, and in another constitutes the stay of the rest of the entablature, and it is from this latter again that itself receives the necessary relation of being a support, progressive architecture also places in external relief this twofold aspect of the main beam by emphasizing in the upper portions of the aspect of support by means of jutting plinths and so forth. In this respect therefore the main beam is not merely related to the columns which support it, but in like degree to other burdens which repose upon it. (_ββ_) These in the first instance constitute the _frieze._ The border or frieze consists in one part of it of the tops of the joists[87], which rest on the entablature, in another part of the spaces between the same. For this reason the frieze contains more essential differences than those distinguishing the architrave, and is bound to emphasize them more sharply, especially in the case where architecture, although executed in stone materials, follows more stringently the fundamental type of the wood construction. This is supplied us by the distinction between triglyphs and metopes. In other words triglyphs are the tops of the beams which are divided into three spaces, the metopes are the rectangular spaces between the separate triglyphs. In former times they were in all probability left bare, in later, however, they are filled up[88], nay, even covered over and decorated with reliefs.