Flying Buttress is an arched exterior support that transfers a roof's outward thrust to the ground.
A flying buttress is an arched, freestanding structure that carries the lateral thrust of a vaulted ceiling or roof away from a building's walls and down into a pier planted some distance from the structure itself. The device let Gothic masons do something Romanesque builders could not: replace thick, load-bearing walls with soaring expanses of stained glass. Notre-Dame de Paris, Chartres, and Reims all owe their height and luminosity to rings of flying buttresses hidden — and sometimes proudly displayed — around their choirs and naves. Because the buttress absorbed the thrust that would otherwise have to be resisted by wall mass, architects could push vaults higher and windows wider with each generation, a competitive escalation that produced the tallest cathedrals in medieval Europe.