This plan shows two levels of the choir hemicycle: the darkly-shaded elements constitute the uprights of the flying buttresses built by Master Renaud in the 1250-60s which sit on top of the lower buttresses between the chapels installed by Master Thomas more than a decade earlier. You can see that the younger master has not continued the solid rectangular form on the main buttresses, preferring a much daintier "Y" shaped unit. Renaud has then placed intermediary uprights (marked D on the plan) atop the piers dividing the chapels.