In the course of the centuries the Roman basilica was modified by the Catholic liturgy. For catechumens, or penitents, was made the porch, or narthex, before the western end. Tribunes were built over the side aisles.[5] Increased church ceremonial brought about a development of the choir. The custom of burying the dead in crypts under the main altar originated the raised chancel. Between the choir and the nave the builders began to insert a transverse nave called a transept.[6] Such an enlargement enabled the congregation to approach closer to the altar ceremonies; only the bigger churches built transepts in the XI century. Then the liturgical writers saw in a transept the extended arms of the Cross, and it was in that spirit the XIII-century transepts were made--their symbolism was posterior. The first ambulatories were no doubt built in churches which possessed some revered relic, to facilitate the passage of the pilgrim crowd. (The term ambulatory will be used to designate the continuation of the choir aisle round the apse.) Before long that curving processional path, with radiating apsidal chapels opening from it, was taken to represent the crown of thorns about the Sacred Head. "All things as pertain to offices and matters ecclesiastical be full of divine signification and mysteries, and overflow with a celestial sweetness: if so be that a man be diligent in his study of them, and know how to draw honey from the rock and oil from the hardest stone." So wrote William Durandus, the XIII-century French bishop whose _Rationale_, or treatise on church symbolism, was an inspiration for centuries and, next to the Bible, the most frequently printed book of the older times.[7] Despite a host of additions to the basilica of Rome--transept, ambulatory, a long choir, apse chapels, towers--despite the discarding of the classic orders and of antiquity's use of a veneer of finer stone (the Romanesque builder used the unadorned stone of his own region) the church of western Europe remained, in general plan, a Roman basilica. Like Rome, they covered their main vessel by a flat wooden roof, although they knew how to build barrel and groin vaulting.[8] Now a wooden roof is an easy prey for fire. Such roofs, a succession of long-continued invasions, and the faulty construction of Merovingian and Carolingian churches are accountable for the fact that in France to-day is no church that predates the year 1000. Some portions of ancient wall are embedded in later work, and some few early crypts are intact. But to speak with certainty of Merovingian and Carolingian architecture is impossible, though they formed the incubating phase of Romanesque art.