FLYING BUTTRESS, in architecture, a term given to an arched strut which transmits the thrust of an arch or vault across an open space, such as a side aisle or chapel, to an independent, vertical buttress built in the line of the outer wall of the space. Certain late Roman buttresses, as in the Baths of Diocletian (305) are pierced by arches and are, therefore, in essence, flying buttresses. It was, however, only towards the middle of the 12th century that the structural and aesthetic possibilities of the flying buttresses were understood; their development from then on be came one of the important features of Gothic architecture (q.v.). The typical Gothic flying buttress consists of a half arch abutting, at its apex, against the nave wall and at the outer end against the vertical mass of the buttress proper, which was frequently weighted by a heavy pinnacle. Where the nave wall was of great height, two, or even three, half arches, one over the other, form the flying buttress.