What external support system is a hallmark of Gothic architecture?

The external support system that became a hallmark of Gothic architecture is the flying buttress. It uses a masonry arch, or flyer, to carry the outward thrust of a roof or vault to a separate external pier.

This structural detour transformed cathedral design. Stone vaults exert lateral pressure on their supporting walls; without relief, those walls had to be extremely thick. Flying buttresses redirected the force outward and downward, allowing architects to build taller interiors with thinner walls and much larger windows filled with stained glass.

The system developed from earlier Roman, Byzantine, and Romanesque precedents, but it flourished in Gothic architecture from the 12th century onward. The flying buttresses of Notre-Dame de Paris, begun around 1180, are among the earliest famous examples associated with a Gothic cathedral. Outer piers were often topped with pinnacles, whose weight helped stabilize the structure.

A regular pier is a vertical support, while a pointed arch and rib vault are other Gothic design elements rather than external support systems. Some later buildings retained flying buttresses partly for visual effect, even when modern structural changes reduced their practical necessity.