The flying buttresses are the architectural feature that made possible the exceptional height of the walls of the nave and choir. The arched buttresses leap over the outer, lower level of the cathedral, where the ambulatory and chapels are located, to strengthen the upper walls of the clerestory. They counteract the outward and downward thrust of the vaulted ceiling, so that the walls between the buttresses can be thin and filled with large windows. The buttresses were later given additional strength by the placement of heavy stone pinnacles on top of their bases.