West side of upper north transept, clerestory and flying buttresses

It is on the west side of the upper transept that the openwork flying buttress first appears. Instead of a massive brace of solid masonry formed by a segment of an arch at the bottom and a rim at the top, we now see tracery panels linking arch and rim. In other words, architectural forms that had previously been considered appropriate for non-structural window tracery are now applied to the serious business of providing structural support. This is part of the revolutionary new architecture characteristic of young Renaud de Cormont, c.1250. The lower arched element of the flyer meets the buttress between the clerestory windows at an awkward angle: it butts against a strip of solid masonry identical to the forms used in the nave possibly already installed by Master Thomas. Solid flyers had, at first, been intended here.