Hi Pilou,

> Let point any external point of the curve as Origine
> click the Xaxe and move/point to the "oposite point" of the first external point
> click the Zaxe and move/point to the "vertical" of the first external point
> that's all ;)

Yup, this method will work, the only thing is after you have placed the x axis to be in the curve's plane, you will then want to do a single click on the x axis to mark it as the "primary axis" (you will see the little round end marker appear at its end), before you start dragging the z axis.

That will make the next drag of the z axis to be forced to only pivot around the x axis rather than it having free movement.

But it may be a little easier to draw in a circle as I mentioned above because then you don't have to do these rotations, snapping to the center point of the circle will place the z axis in its correct alignment from the start.

- Michael