A pile of masonry built at right angles to a wall to strengthen it at certain points to resist the thrust of vaulting. Its extensive use on the exterior of ecclesiastical buildings became necessary when the medieval builders substituted vaulting for wooden trabeated roofs, and variations ranged from massive supports embedded in the wall to graceful counter-thrusts touching it only at points. The flying buttress, which became the most distinctive characteristic of Gothic architecture, originated in France in the 12th century when ribbed vaulting was extended to the nave. It consists of a straight band of stone covered by a half-arch which transmits the thrust of a vault across an open space to a pier independent of the main structure. In five-aisled churches it sometimes crosses both aisles in a single span (Notre Dame, Paris) and sometimes in two spans (Rheims; Amiens).