The next three schools of Romanesque architecture have one feature in common, namely, the employment of groined vaults over the side aisles. But the form which these assume and their relations to the tunnel vaults of the nave differ sufficiently to distinguish the churches of Poitou, Auvergne and Bourgogne from one another. [Illustration: FIG. 11.--SOUVIGNY, ABBEY CHURCH.] THE SCHOOL OF POITOU The chronology of the churches of Poitou is somewhat obscure, but the vaulting principles of the school were well developed early in the eleventh century, to which period a number of the existing churches belong. Their naves are tunnel vaulted and without a clerestory, the light entering through windows in the outer walls of the aisles, which are narrow and high and covered with groined vaults rising from the imposts of the arches opening into the nave. The entire church has a single-gabled exterior roof of wood and tile, its rafters supported near their centers by a wall above the nave arcade, and thus not resting directly upon the extrades of the vaults.[60] Certain minor structural differences make it possible to divide the churches of Poitou into two groups. The first is composed of the earlier churches, of which Saint Savin-sur-Gartempe (Vienne) (begun cir. 1023) is the best and perhaps the only existing example. In it, both nave and aisle vaults are without transverse arches. All the vaults are semicircular in section, and those of the aisles[61] have their transverse surfaces continuous with the soffits of the nave arches.[62] This gives them the flattened groins so characteristic of Roman architecture. Such a system as this required an extensive wooden centering, and it is not surprising that the builders of Poitou soon introduced transverse arches beneath the vaults,--perhaps through the influence of Lombardy, where they were in use as early as the tenth century[63]--thus producing a group of churches which form the second type of the school. Notre Dame-la-Grande at Poitiers (Vienne) (early twelfth century), is an early example of this class. Transverse arches are employed throughout the church, not only strengthening the vaults but making it possible to save centering by using the same form for each successive bay and at the same time reducing to some extent the thickness of the web by thus breaking it up into smaller units.[64]