This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol5", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
In the Classic and Renaissance varieties of doorways the heads are usually horizontal, or in other words a "lintel" is inserted to form the head, with a relieving arch to throw the weight of the superincumbent wall on to the abutments. In many doorways of this style the arch is made a feature and turned over as a semicircle, with moulded architraves and other ornamentation. Where this type is used and the intervening space is filled in with a window, the lintel becomes distinct from the arch, and does not serve as a support. It is then termed a "transome," acting then as a bond stone instead of a weight carrier.
In modern work doorways are frequently ornamented by pilasters and columns, with entablatures, tympanums, and pediments complete, partaking of the general characteristics of the style of Classic architecture used in the work.
It is a feature in modern construction frequently to case the heads with stone in imitation of a lintel composed of several distinct voussoirs jointed together.
The backing in these cases, where necessitated by large openings below, is composed of massive steel girders, which take the weight of the superstructure and thus relieve the lintels of any strain.
It has already been stated, the main distinguishing features of Greek and Roman mouldings and their modern equivalents, are their adaptation from sections of the cone, i.e. the parabola, hyperbola, and ellipse in the first case, and from the circle in the second.
In the Grecian style the distinctive feature is the great beauty of the curves produced by the application of the conic section to the mouldings in use therein, as illustrated in Fig. 144. The reference letters indicates the names of the mouldings: - a, cavetto; b, scotia; c, cyma recta; d, cyma reversa; c, echinus; f, ovolo; g, torus. The Roman forms of these, similarly lettered, are shown in Fig. 145.
From these and their combinations the majority of modern mouldings are obtained, coupled with plane faces and chamfers.