Hyperbolas sharing common foci (left figure). The family of confocal hyperbolas covers the plane simply, in the
sense that there is a unique hyperbola passing through each point in the plane (Hilbert
and Cohn-Vossen 1999, p. 5). The figure on the right shows confocal hyperbolas
superimposed on confocal ellipses, which form
an orthogonal net of curves (Hilbert and Cohn-Vossen 1999, pp. 5-6).
