"A surface which may be generated by a moving straight line is called a '. The plane, the cone, and the cylinder are simple examples... the hyperbolic paraboloid is a . ...[T]he unparted hyperboloid \frac{x^2}{a^2} + \frac{y^2}{b^2} - \frac{z^2}{ c^2} = 1 is a ruled surface having two sets of rectilinear generators, i.e., ...through every point of it two straight lines may be drawn, each of which shall lie entirely on the surface."