The mirrors are usually a section of a rotated parabola and a hyperbola, or ellipse. There are three types of conics&colon; the ellipse, parabola, and hyperbola. These correspond respectively to the limiting forms of an ellipse, parabola, and a hyperbola. They cover the ellipse (50 propositions), the parabola (22), and the hyperbola (28). A parabolic segment is the region bounded by a parabola and line. A parabolic arch is an arch in the shape of a parabola. This is the equation of a parabola, so the path is parabolic. The envelope of the directrix of the parabola is also a catenary. Therefore, is the tangent to the parabola at P. For a parametric equation of a parabola in general position see . The axis of the parabola is vertical. It has a chord , which joins the points where the parabola intersects the circle. By symmetry, F is on the axis of symmetry of the parabola. It is a surface of revolution obtained by revolving a parabola around its axis. But that is a routine consequence of the equation of the parabola.