Thanks Micheal and Burrman,

In fact the isocurve is similar to what I thought but not exactly :)
And I don't want to work like a subd model, just wanted to modify some shape. I didn't know that Tspline was just a way to convert your subd in Nurbs.
What I wanted to do is having a kind of freeform modeling without loosing the power of working with curves.
Instead of modifying the surface points you change the construction curves.

For example in the picture I made the shape with sweep, what I had in mind is that if I modify any of these curves after the sweep is done, the other curves follow the modification to fit.
At the moment I can do it manually and it allow me to have simple shape construction and a great amount of control while still having perfect surface curvature.

Hi David,

> For example in the picture I made the shape with sweep, what I had in mind is that if I modify any of
> these curves after the sweep is done, the other curves follow the modification to fit.

Well, you can modify the curves right after doing the sweep, and have the sweep surface itself recalculate and update, but it's the generated surface that will get recalculate, not other curves.

It's not really clear to me exactly how you would expect the other curves to update when you change some other curve - unlike the surface that is generated from curves the other curves are usually just drawn in each individually and don't usually have any kind of special connection to one another...

- Michael