Absolutely no offence taken - it was indeed in my (mis)understanding of what I was seeing. It is now much clearer, and your clarification just sets out very well what I clumsily sort-of-worked-out by experiment and observation in a variety of cases.
I think I was further confused when Chris's plugin seemed to be getting the same unexpected results as I was, and by a rotation I was doing of a view.draw_polyline about the reported face.normal which seemed to be visually wrong - it was actually rotating the polyline in the face plane, about an axis apparently NOT visually normal to it! However, since the only thing that was really bothering me was that when I tried to do a view.draw_line for the normal, it was in the wrong direction, I now understand why, and have fixed it.
Thanks again for your helpful comments.
Has anyone ever done a Tutorial about programming Ruby Tools in SU? I haven't found one, and have done almost all my learning from reading the sample code for CLineTool, and reading and re-reading the API documentation, which is very thin on real world examples and sample outputs. It's been hard work, though ultimately I've got something that now works for rectangular profiles.
Next challenge - to extend it to arbitrary profiles of mouldings. A bit like a simpler version of the Profile Builder plugin from Dale Martin and SMustard - indeed, perhaps in comparison just a poor thing, but mine own, as one of Shakespeare's characters says.
