Before the A-12, titanium was used only in high-temperature exhaust fairings and other small parts directly related to supporting, cooling, or shaping high-temperature areas on aircraft like those subject to the greatest kinetic heating from the airstream, such as wing leading edges. The A-12, however, was constructed mainly of titanium. Titanium is quite rigid and difficult to machine, which made it difficult to form into curves given available techniques. This made it difficult to form the leading edges of the wing and similar surfaces. The solution was found by machining only small "fillets" of the material with the required shape and then gluing them onto the underlying framework which was more linear. A good example is on the wing; the underlying framework of spars and stringers formed a grid, leaving triangular notches along the leading edge that were filled with fillets.