Q is another point on the parabola, with perpendicular to the directrix. The envelope of the directrix of the parabola is also a catenary. One description of a parabola involves a point (the focus) and a line (the directrix). Conversely, two tangents which intersect on the directrix are perpendicular. Hyperbolas share many of the ellipses' analytical properties such as eccentricity, focus, and directrix. The catenoid is obtained by rotating a catenary about its directrix. The focal parameter () is the distance from a focus to the corresponding directrix. The latus rectum is parallel to the directrix. One may consider the directrix of a circle to be the line at infinity. For any cone one can choose the apex as the directrix. De Witt invented the term "directrix". This case shows&colon; "The directrix of a ruled surface may degenerate to a point". The point C is located on the directrix (which is not shown, to minimize clutter). The focus does not lie on the directrix.