The directrix of a conic section is the line which, together with the point known as 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, with
being the constant of
proportionality. If the ratio 
,
the conic is a parabola, if 
,
it is an ellipse, and if 
, it is a hyperbola (Hilbert
and Cohn-Vossen 1999, p. 27). Hyperbolas and noncircular ellipses have two distinct foci and two associated directrices, each directrix being perpendicular to the line joining the two foci (Eves 1965, p. 275).
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