Connect unequal surfaces along path

Dear people, I hope someone can help me out with this simple thing.
I’m new to Rhino and need to create a solid connecting a square with a rectangle along a path following half a circle.
I have a picture of the object (a sculpture) but I’m not sure if I can upload it.
Can anyone explain me the basic principle how to connect unequal surfaces along a path?
Not sure if my question is clear. Hope it is.
Thanks in advance.
Best wishes,


Hi Rutger,

I’m trying to visualise your 3D object but i think I know what you mean… Here is an example with no twist.

I’ve used the command ‘sweep 1 rail’ from the surface drop down.