The Gothic idea born at Saint-Denis becomes, in Chartres Cathedral rebuilt after the fire of 1194, a mature and systematic structure. The lesson of Chartres lies in one element: the flying buttress.

The Gothic vault gathers its load onto points through the ribs; but at those points there is still a great lateral thrust — the vault tries to push the walls outward. The Romanesque solution was to thicken the wall. Chartres instead throws the thrust outside the building: stone piers rising apart from the main wall, and sloping arches reaching from them to the vault's thrust point — flying buttresses — catch the thrust and carry it to the ground. The support is now not within the wall but outside it.

The result is something architecture had not seen until then: a true skeletal structure. The load is carried by piers, ribs, and flying buttresses; the wall between them keeps no load-bearing role. At Chartres these gaps are filled from top to bottom with stained glass — the side walls are no longer stone but surfaces woven of light. Its famous cobalt-blue glass floods the interior with a shimmering coloured light. Chartres thus shows the core principle of Gothic completely: reduce structure to a skeleton and the wall is freed, and the absence of wall turns into light.