It may excite surprise that what appears to be so small a difference as that which exists between a beam, a round arch, or a pointed arch, should be employed in order to distinguish three of the four great divisions. But in reality this is no pedantic or arbitrary grouping. The mode in which spaces or openings are covered lies at the root of most of the essential differences between styles of architecture, and the distinction thus drawn is one of a real, not of a fanciful nature. Every building when reduced to its elements, as will be done in both these volumes, may be considered as made up of its (1) floor or plan, (2) walls, (3) roof, (4) openings, (5) columns, and (6) ornaments, and as marked by its distinctive (7) character, and the student must be prepared to find that the openings are by no means the least important of these elements. In fact, the moment the method of covering openings was changed, it would be easy to show, did space permit, that all the other elements, except the ornaments, were directly affected by the change, and the ornaments indirectly; and we thus find such a correspondence between this index feature and the entire structure as renders this primary division a scientific though a very broad one. The contrast between the trabeated style and the arched style may be well understood by comparing the illustration of the Parthenon which forms our frontispiece, or that of the great temple of Zeus at Olympia (Fig. 4), with the exterior of the Colosseum at Rome (Fig. 5), introduced here for the purposes of this comparison. [Illustration: FIG. 4.--TEMPLE OF ZEUS AT OLYMPIA. RESTORED ACCORDING TO ADLER.]