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