The second great group of buildings is that in which the semicircular arch is introduced into construction, and used either together with the beam, or, as mostly happened, instead of the beam, to span the openings. This use of the arch began with the Assyrians, and it reappeared in the works of the early Etruscans. The round-arched series of styles embraces the buildings of the Romans from their earliest beginnings to their decay; it also includes the two great schools of Christian architecture which were founded by the Western and the Eastern Church respectively—namely, the Romanesque, which, originating in Rome, extended itself through Western Europe, and lasted till the time of the Crusades, and the Byzantine, which spread from Constantinople over all the countries in which the Eastern (or Greek) Church flourished, and which continues to our own day. The third group of buildings is that in which the pointed arch is employed instead of the semicircular arch to span the openings. It began with the rise of Mohammedan architecture in the East, and embraces all the buildings of Western Europe, from the time of the First Crusade to the revival of art in the fifteenth century. This great series of buildings constitutes what is known as pointed, or, more commonly, as gothic architecture. The fourth group consists of the buildings erected during or since the Renaissance (_i. e._, revival) period, and is marked by a return to the styles of past ages or distant countries for the architectural features and ornaments of buildings; and by that luxury, complexity, and ostentation which, with other qualities, are well comprehended under the epithet modern. This group of buildings forms what is known as Renaissance architecture, and extends from the epoch of the revival of letters in the fifteenth century to the present day. The first two of these styles occupy those remote times of pagan civilization which may be conveniently included under the broad term ancient; and the better known work of the Greeks and Romans—the classic nations—and they extend over the time of the establishment of Christianity down to the close of that dreary period not incorrectly termed the dark ages. 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.