Hi Pilou,

> that's this! I have tollally missed this paradigmatic thing!!!
> It's an Earthquake for me! :)

Yeah it's one of the big differences with NURBS as compared to polygon modeling - NURBS can have Trim curves that trim away parts of an "underlying surface".

There's some more description of this here:

http://moi3d.com/wiki/FAQ#Q:_Why_does_show_points_work_for_some_objects_but_not_others.3F


> From what comes the measure of the length? I have only half of the form!

It just comes from whatever was the original surface before it was trimmed or booleaned.

So in this case for some reason the original full surface was trimmed by only half of the curve outline, so when removing the trims it restores the original full part, which is convenient for this case.

- Michael