Hi Michael and Jonah,

Thanks for the info. It's quite clear now (apart the terms "rate of change" in the explanation - from Rhino - for G3 and G4 which remain "obscure").
The comments on Bezier curves here and in another thread are very useful too.

Now, just for a better understanding of NURBS in general...
Very often I see mention of a degree for NURBS. Could you explain what that is ? Is there a relation between the "G value" of a curve and its degree or is this totally different stuff ?
No need for a deep technical explanation there but I think understanding what's "behing the engine" always permits a better understanding of the software and generally speaking why sometimes the results are not exactly what the user expected.
Thx