In most modern paragliders (from the 1990s onwards), some of the cells of the leading edge are closed to form a cleaner aerodynamic profile. Holes in the internal ribs allow a free flow of air from the open cells to these closed cells to inflate them, and also to the wingtips, which are also closed.[12] Almost all modern paragliders follow a sharknose design of the leading edge, by which the inflation opening is not at the front of the wing, but slightly backwards on the underside of the wing, and following a concave shape. This design, resembling the nose of a shark, increases wing stability and stall resistance.[13][14] In modern paragliders, semi-flexible rods made out of plastic or nitinol are used to give extra stability to the profile of the wing. In high-performance paragliders, these rods extend through most of the length of the upper wing.