North flank of upper nave, seen from north transept

Here you can see the massive pylons (culées, in French--there is no very good word in English) that launch double-rank flying buttresses to support the high vaults and to help brace the upper wall and roof. The great clerestory windows contain decorative tracery in the form of a cusped oculus surmounting four lancets grouped in pairs, each with a smaller oculus. The same pattern continues without change down the length of the nave. Around 1500 the outer edges of the culées were consolidated with the insertion of new pinnacles each designed around a square from which emerges a second square, diagonally turned. The rotational generation of form is one of the hallmarks of Gothic. The great buttresses originally continued on down to the ground; in the 14th century they were extended outwards and lateral chapels were added: the buttresses now appear to grow out of the chapel roof.