The model shows the meridian curve of a surface of revolution. Those parts of the meridian that generate elliptic points of the surface are coloured green, those that generate hyperbolic points are marked red. Points where the meridian tangents are parallel to the axis of rotation produce equatorial circles or gorge circles, depending on whether those points are elliptical or hyperbolic. The generating points of these circles are marked by green or red spheres respectively. Along the equator and gorge circles cylinders of revolution the surface of revolution can be circumscribed. The transition between the elliptic and the hyperbolic points is generally brought about by parabolic points. In a meridian these are usually points of inflexion or extrema (marked by white spheres).