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A surface such that for every point there is a straight line passing through it. The family of lines are called a ruling of the surface. The points used as initial points in some parametrization are called directrix.

• Plane (surface)
• Cylinder
• Cone
• Hyperboloid of revolution (1-sheet)
• Tangent surface to a curve
• Helicoid

The local parametrizations used are in general not parametric..

More about ruled surfaces, deriving the curvatures, etc.

The Gaussian curvature is $\leq 0$. If it is 0, it is called developable. vid. They are either (general) Cylinders, Cones, or tangent surfaces to curves.