+STRUCTURAL PRINCIPLES.+ All architecture is based on one or more of three fundamental structural principles; that of the _lintel_, of the _arch_ or _vault_, and of the _truss_. The principle of the +lintel+ is that of resistance to transverse strains, and appears in all construction in which a cross-piece or beam rests on two or more vertical supports. The +arch+ or +vault+ makes use of several pieces to span an opening between two supports. These pieces are in compression and exert lateral pressures or _thrusts_ which are transmitted to the supports or abutments. The thrust must be resisted either by the massiveness of the abutments or by the opposition to it of counter-thrusts from other arches or vaults. Roman builders used the first, Gothic builders the second of these means of resistance. The +truss+ is a framework so composed of several pieces of wood or metal that each shall best resist the particular strain, whether of tension or compression, to which it is subjected, the whole forming a compound beam or arch. It is especially applicable to very wide spans, and is the most characteristic feature of modern construction. How the adoption of one or another of these principles affected the forms and even the decoration of the various styles, will be shown in the succeeding chapters. +HISTORIC DEVELOPMENT.+ Geographically and chronologically, architecture appears to have originated in the Nile valley. A second centre of development is found in the valley of the Tigris and Euphrates, not uninfluenced by the older Egyptian art. Through various channels the Greeks inherited from both Egyptian and Assyrian art, the two influences being discernible even through the strongly original aspect of Greek architecture. The Romans in turn, adopting the external details of Greek architecture, transformed its substance by substituting the Etruscan arch for the Greek construction of columns and lintels. They developed a complete and original system of construction and decoration and spread it over the civilized world, which has never wholly outgrown or abandoned it. With the fall of Rome and the rise of Constantinople these forms underwent in the East another transformation, called the Byzantine, in the development of Christian domical church architecture. In the North and West, meanwhile, under the growing institutions of the papacy and of the monastic orders and the emergence of a feudal civilization out of the chaos of the Dark Ages, the constant preoccupation of architecture was to evolve from the basilica type of church a vaulted structure, and to adorn it throughout with an appropriate dress of constructive and symbolic ornament. Gothic architecture was the outcome of this preoccupation, and it prevailed throughout northern and western Europe until nearly or quite the close of the fifteenth century.