An architectural term borrowed by tube-bending to describe the geometry of material flow. The extrados is that section of the tube which is undergoing elongation during the bend forming process; that part of the tube of which the wall thins. Roughly, the extrados corresponds to outboard half of the tube bend, but in fact includes a significant portion of the tangents and some of the tube inboard of the centerline radius. The extrados is separated from the intrados by the neutral axis.