Notre-Dame de Paris is the building in which the Gothic logic born at Saint-Denis was tested at large scale and the flying buttress became not a façade motif but a structural organ pushed outside the building.

The Gothic problem is this: the rib vault gathers its load onto a few points — but at those points it produces a strong thrust not only downward but sideways. As the vault span grows and the wall thins, meeting this lateral thrust with a thick wall inside becomes impossible. The solution is outside: to 'fly' the thrust out of the building on an arch and carry it to a free-standing buttress pier some distance away — the flying buttress. The wall then no longer bears the load; the load is caught by an external ring of stone arms and brought down to the ground. The emptied place of the wall is given over to vast stained-glass windows.

Notre-Dame is among the first cathedrals to use this system at large scale and visibly for its high vaults; the fan of flying buttresses encircling the apse is a moment in which architecture no longer hides the forces that hold up its interior but displays them outside. Structure comes before decoration and shows itself. The 2019 fire destroyed the roof and spire but the stone vault-and-buttress skeleton largely stood — itself a proof of the soundness of the building's central structural idea. Notre-Dame is one of the urban-scale manifestos of the Gothic ideal of a 'cage of stone full of light'.