Gothic: Rib Vault, Pointed Arch, Flying Buttress

A round Romanesque arch pushes its load out sideways as much as down, so the wall beneath it has to be a thick mass of stone just to resist being shoved outward. A pointed arch changes the geometry: its two curves meet at a steep angle at the top, which sends more of the load straight down through the piers and less of it sideways. Less sideways thrust means less wall is needed to resist it.

A rib vault makes this concrete. Masons first build stone ribs, arched diagonals and crossing lines that form a skeleton over each bay. Only after the ribs are standing do workers fill the gaps between them with thin stone webbing, almost a shell, since the webbing carries little weight and the ribs do the structural work. The ribs funnel the vault's thrust down to specific points on the piers rather than spreading it along the whole wall, which is why a Gothic nave wall can be thin and full of openings where a Romanesque wall could not.

The thrust still has to go somewhere, and that is the flying buttress's job: an arched bridge of stone that catches the outward push at the top of the nave wall, carries it over the side aisle roof, and lands it on a heavy exterior pier. With thrust routed outside, the nave wall is freed from carrying structural load and becomes mostly glass. Chartres shows the full result, stacking arcade, triforium, and clerestory into one tall elevation whose glowing windows are only possible because the engineering happens outside the wall, not inside it.