Developable Surface --Computer Graphic

What extractly the Developable Surface is?Here's the answer:

Intuitively, developable surfaces are those that can be un-folded into the plane with no length or area distortion.Developable surfaces are closely related to ruled surfaces,
where a ruled surface is defined as the trajectory of a line(called a ruling ) when it moves in space along two pre-scribed directrix curves .

Bézier&& B-spline surface(贝塞尔曲面和B-样条曲面) Google..

My Job is to use the developable surface to sulimulate cloth motivation with the algorithm i put  up .And the developable is enforced by nonlinear constraints,using the surface as above.