# directrix

The directrix is the line that, together with a point called the focus,
serves to define a conic section as the locus of points whose distance from the focus is proportional to the horizontal
distance from the directrix. If the ratio *r* = 1, the conic is a parabola, if *r* < 1, it is an ellipse,
and if *r* > 1, it is a hyperbola.