A surface that contains two families of rulings. The only three doubly ruled surfaces are the plane, hyperbolic paraboloid, and single-sheeted hyperboloid.

Hilbert, D. and Cohn-Vossen, S. Geometry and the Imagination. New York: Chelsea, p. 15, 1999.

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Weisstein, Eric W. "Doubly Ruled Surface." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/DoublyRuledSurface.html