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The churches in the third Provence group differ from those in the second only in having half tunnel vaults in the side aisles, but this difference is sufficient to change to some extent the character and methods of construction. In the simple churches of this type where there is no clerestory as, for example, in the western portion of the little church of Saint Honorat, belonging to the monastery of the Isle-de-Lérins (Alpes-Maritimes),[43] the half tunnel vault of the aisles furnishes better abutment for that of the nave than the full tunnel vaults of the second type, and at the same time permits loftier arches to be constructed in the nave arcades, giving a better distribution of light without raising the imposts of any of the vaults.

When, however, a clerestory is added, as in Saint Trophime at Arles (first half of the twelfth century), the inward pressure of the aisle vaults is even more severe than in Saint Guilhem-du-Désert and at the same awkward place, so that the only structural advantage at Arles lies in the added height of the nave arches. It is a noticeable feature of Saint Trophime that the aisles have full, instead of half arches[44] used transversely beneath the vaults, very probably because the former exerted less inward thrust, and could also be weighed down by a solid wall which increased the rigidity of the structure by tying the pier of the nave arcade to the outer wall, and strengthened the clerestory for the support of the high vault. The system has already been noted in the cathedral of Molfetta,[45] and will be found repeated either in the triforia or aisles of a number of Romanesque churches of different schools.[46]