In Early Gothic architecture, following the model of the Romanesque churches, the buildings had thick, solid walls with a minimum of windows in order to give enough support for the vaulted roofs. An elevation typically had four levels. On the ground floor was an arcade with massive piers alternating with thinner columns, which supported the six-part rib vaults. Above that was a gallery, called the tribune, which provided stability to the walls, and was sometimes used to provide seating for the nuns. Above that was a narrower gallery, called the triforium, which also helped provide additional thickness and support. At the top, just beneath the vaults, was the clerestory, where the high windows were placed. The upper level was supported from the outside by the flying buttresses. This system was used at Noyon Cathedral, Sens Cathedral, and other early structures.