+STYLE+ is a quality; the “historic styles” are phases of development. _Style_ is character expressive of definite conceptions, as of grandeur, gaiety, or solemnity. An _historic style_ is the particular phase, the characteristic manner of design, which prevails at a given time and place. It is not the result of mere accident or caprice, but of intellectual, moral, social, religious, and even political conditions. Gothic architecture could never have been invented by the Greeks, nor could the Egyptian styles have grown up in Italy. Each style is based upon some fundamental principle springing from its surrounding civilization, which undergoes successive developments until it either reaches perfection or its possibilities are exhausted, after which a period of decline usually sets in. This is followed either by a reaction and the introduction of some radically new principle leading to the evolution of a new style, or by the final decay and extinction of the civilization and its replacement by some younger and more virile element. Thus the history of architecture appears as a connected chain of causes and effects succeeding each other without break, each style growing out of that which preceded it, or springing out of the fecundating contact of a higher with a lower civilization. To study architectural styles is therefore to study a branch of the history of civilization. Technically, architectural styles are identified by the means they employ to cover enclosed spaces, by the characteristic forms of the supports and other members (piers, columns, arches, mouldings, traceries, etc.), and by their decoration. The +plan+ should receive special attention, since it shows the arrangement of the points of support, and hence the nature of the structural design. A comparison, for example, of the plans of the Hypostyle Hall at Karnak (Fig. 11, h) and of the Basilica of Constantine (Fig. 58) shows at once a radical difference in constructive principle between the two edifices, and hence a difference of style.