Hi ksiksu,

> Is there a mistake in the construction logic?

Sort of, yes - the problem is you've got such a long single segment curve that has those tight little corner areas in it.

It's difficult for a surface to be created between 2 such curves with nice alignment between those corner areas.

Because these are just long splines the surfacing command doesn't really know that you would like just the corner areas in particular to exactly correspond with each other, it will just kind of make connections between the surfaces using things like equal distance traveled along each of them, which makes for connections like this (here I've drawn in connection lines in red):





So that's what is producing that kind of slight twisting effect - connections happening between the curves at different locations than having each corner connected to the other corner only.

The way to solve this is to make your original curves made out of some different segments with one segment for the long straight piece and other segments for the corners. Then with 2 curves made out of an equal number of segments between them a surfacing command (like Loft for example which is probably what you'd want to use here) will know to make connections from each segment to the other segment, that will make the corners match up well.

I'll see if I can give an example.

- Michael