Every Gothic cathedral fights lateral thrust, the outward push created when a heavy stone vault presses down on thin walls. The flying buttress solved it with two parts: a freestanding pier anchored in the ground and an arched flyer that carries the force across open air. Stacked flyers handled both the vault and the wind hitting timber roofs, while the pinnacles on top acted as heavy paperweights steering the thrust down into the foundations. Building them meant hoisting wooden centering frames by treadwheel crane and holding the stones still while lime mortar slowly carbonated back into limestone.

Hidden ancestors appear at the Rotunda of Galerius in Thessaloniki and San Vitale in Ravenna, and Romanesque builders at Durham tucked them under gallery roofs. In the 1160s, around Paris, they moved outside, at Saint-Germain-des-Prés in 1163 and Notre-Dame around 1180. Early flyers at Chartres were bulky, later ones at Amiens and Le Mans were a single voussoir thick, and Beauvais pushed too far when wind snapped its slender supports in 1284. The Renaissance dropped the design, but it returned as a crutch for failing walls and, in the 20th century, on Canadian lighthouses.

• The barbell analogy: a friend pushing inward on your shoulders is what a flyer does for a cathedral wall
• Why the lower flyer sat below the vault’s springing point and the upper flyer took the wind
• How the 1284 windstorm at Beauvais brought down a choir vault soaring over 150 feet
• A flying buttress added purely as a repair to a buckling church wall in Chaddesley Corbett, Worcestershire
• William P. Anderson’s concrete flying buttresses on the lighthouse at Pointe-au-Père, and Ken Follett’s The Pillars of the Earth