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Fig. 5 (Left) for a conical surface, all rulings intersect in a common apex; (right) a general ruled surface does not fulfill this property. For an arbitrary point on either of the colored curves, the directions of its tangent and the intersecting ruling are called conjugate.

The tangent surfaces of space curves, general cylinders and general cones are subsumed in the class of developable surfaces. They are a subclass of ruled surfaces i.e. surfaces that carry a family of lines, with the special property that the tangent plane along a ruling does not change its direction, see (Do Carmo 1976).