Gothic cathedrals got their huge windows by pushing their supports outside.
A stone vault pushes its walls outward, so early churches needed thick walls with small windows. The flying buttress, an arch leaping from the upper wall to a heavy pier standing away from the building, carried that thrust across open air. Walls could be thinner, windows vast, and the stained-glass cathedral was born.
A stone vault is heavy, and it doesn't only press down; it also pushes sideways, trying to spread the walls apart. Wind on a high roof adds more sideways force. The traditional answer was a buttress, a mass of masonry built against the wall from top to bottom. The flying buttress does something cleverer: a ramping arch, the 'flyer', springs from the upper part of the wall and carries the thrust across open space to a massive pier standing some distance away.
The idea had ancestors. Late antique buildings such as San Vitale in Ravenna used early versions, and Romanesque churches like Durham Cathedral hid similar arches under the roofs of their side aisles. By the 1160s, builders around Paris were running longer, finer arches out over the aisle roofs, where they could be seen. The flying buttresses of Notre-Dame de Paris, from about 1180, were among the earliest on a Gothic cathedral.
Engineering refined it. Many Gothic churches used two flyers, one above the other: the lower to resist the thrust of the vault, the upper to resist wind on the roof. Pinnacles on top of the piers added weight to help resist the sideways push. Early examples, like those at Chartres around 1210, were heavier than needed; later builders at Amiens and Beauvais thinned them dramatically. Each arch was built over temporary wooden frames, called centring, until the mortar cured.
The payoff was light. With the thrust carried outside, walls no longer needed such mass, so they could be opened up into tall windows filled with stained glass, and the interiors soared. What began as structural necessity became the signature look of Gothic architecture. Renaissance builders abandoned it in favour of thick walls, but an early 20th-century Canadian engineer revived it to build lighthouses.
Source: Wikipedia — Flying buttress · Text summarised from Wikipedia (CC BY-SA 4.0)
