Skew surface with its tangent paraboloid. String surface model, 1872.

Skew surface with its tangent paraboloid, capable of transformation into another skew surface while the paraboloid deforms into a plane. This is one of a large set of ruled surface models made by Fabre de Lagrange, 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.