Skew surface with its tangent paraboloid, capable of transformation into another skew surface while the paraboloid deforms into a plane; this is (for a certain position of the lower semicircle) a skew surface with a director plane, the plane being vertical

This is one of a large set of ruled surface models made by Fabre de Lagrange, in Paris, France, following designs introduced in the early 19th century. They illustrate surfaces which can be traced out in space by the movement of a straight line.