This portion of the basilica underwent a later modification; from the _presbyterium_ it became the _martyrium_, or shrine in which was placed the body of the patron saint of the basilica or the relic to which the devotion of the faithful was specially addressed. This usage had been established even before the year 500 in the first basilica of St. Martin at Tours. The primitive apse was lighted only from the nave or transept. After its transformation into the _martyrium_ it was not only pierced with windows, but, according to some authors, was provided with openings along its base, or even arcaded, so as to give access to a low gallery running round it. If this be so, the characteristic arrangement of mediæval churches dates from the fifth century. In later times when it became customary to place the altar at the back, against the wall of the apse, seats for the bishops, priests, and choristers--_the choir_--were arranged between the altar and the nave. In monastic churches, built after the Latin tradition, the choir was generally in the crossing, or where there were no transepts, in the nave itself. It was separated from the congregation by a low enclosure of stone or marble. There are a few examples of churches with _two_ choirs, one at the east, the other at the west. In the first churches of the Romanesque epoch the choir was confined to the space between the piers of the crossing; it soon, however, made considerable advances. In monastic churches the choir or sanctuary was cut off from the surrounding spaces by barriers of stone or wood, and towards the nave was closed by a _jubé_, or rood screen and loft, the upper part of which was accessible to the monks for the reading of the epistle and gospel. Bishops, on the other hand, being free from the necessity of closing the choirs of their cathedrals, made a point of providing their flocks with wide spaces, in which ceremonies could be afforded a liberal development. At the end of the twelfth century and beginning of the thirteenth these ideas governed the construction of important churches. Changes continued to be made, however, and from the reign of St. Louis we find the choirs of great cathedrals arranged on the exclusive principles of the monastic churches. The arcades surrounding them were filled with high stone walls, against the inner sides of which the stalls of the clergy, with their lofty and richly carved wooden canopies, were securely fixed. Among the more famous choirs we may quote those of Notre Dame de Paris, of Amiens, of Beauvais, of Auch, of Lincoln, of Canterbury, of Spires, of Worms, of Burgos, etc. In order to satisfy the laymen whose view of the ceremonies performed in the choir was intercepted by these enclosures, the sanctuary was surrounded by chapels contrived in the wall of the apse, and in the side aisles of the nave.