Hi Lemo, I see - but in the case you show here the segments for your curves are not all in the same plane.

Offset is generally much more predictable when done on curves that have all their pieces within a single plane.

As you've seen, it will actually work to do it on ones that are non-planar, but I think you may run into some problems with the corner extensions in that case, and they may need some manual cleanup.

From what I can see from what you are trying to do here, you probably want to edit your curved part so that it has an ending tangent going in a horizontal direction - for example if I take this curve from your file:



When I turn on control points, I see this point structure:




If you look closely you can see that the last 2 points of the curve are not horizontally aligned, they have a slight angle to them:



That means that the curve's tangent is not horizontal either at the end of the curve, it has a angular deviation from horizontal which means that when offset, the offset of the end will travel in that perpendicular direction which will not be exactly vertical and will make it to deviate away from the line segment.

If you edit the last 2 control points (either with the edit frame using the Flat snap, or with Transform/Align) so that they are trued up to be horizontal to one another like this:



Then your offset should work a lot better since the pieces will be tangent to one another and their offsets will naturally touch one another without any extensions being necessary.

I've attached a tuned up version offset_2.3dm which you can try.

Also, one other note on Offset - sometimes the mechanism that detects self-intersections in the offset portion and trims them out can get confused and trim away too much. When that happens, try unchecking the "Trim" option (which shows up in the command options area when doing a curve offset) so that you get the raw offset shape which then you can manually trim as needed.

- Michael