Sec. 1. Side by side with the establishment of Christianity as the religion of the Roman empire, there appeared a fully developed plan for places of Christian worship. The normal Christian church of the fourth century of our era was an aisled building with the entrance at one end, and a semi-circular projection known as the apse at the other. The body of the building, the nave with its aisles, was used by the congregation, the quire of singers occupying a space, enclosed within low walls, at the end nearest the apse. In the apse, raised above the level of the nave, was the altar, behind which, ranged round the wall, were the seats for the bishop and assistant clergy. This type of church, of which the aisled nave and the apse are the essential parts, is known as the _basilica_. The name, employed to designate a "royal" or magnificent building, had long been applied to large buildings, whether open to the sky or roofed, which were used, partly as commercial exchanges, partly as halls of justice. It is still often said that the Christian basilicas were merely adaptations of such buildings to sacred purposes. Some of the features of the Christian plan are akin to those of the secular basilica. The apse with its semi-circular range of seats and its altar reproduces the judicial tribune, with its seats for the praetor and his assistant judges, and its altar on which oaths were taken. The open galleries, which in some of the earliest Christian basilicas at Rome form an upper story to the aisles, recall the galleries above the colonnades which surrounded the central hall of some of the larger secular basilicas. Again, the _atrium_ or forecourt through which the Christian basilica was often approached has been supposed to be derived from the _forum_ in connexion with which the secular basilica was frequently built.