Hi Pilou, it's hard to explain, it is sort of how Rail revolve works.

One part is that rail revolve needs to combine the 2 curves together into a surface. So it is more predictable to position the rail curve so that it is at one edge or at one side of the profile curve rather than running right down the middle of it.

Down the middle would be more how sweep works, which kind of guides the profiles along the path. In rail revolve, the path becomes more directly a part of the surface, it is almost like the path and the profile are multiplied together more directly in a certain sense.

Then the other part is that rail revolve is also a type of revolve which applies scaling outward from the central axis. If your profile curve is sort of slanted with respect to the axis instead of straight in line with the axis, you will also possibly get some less predictable scaling results. So to get more predictable results, place things more like this:



But still since it involves rotation and scaling, you will get different sized results along the closer or farther legs of the rectangle. You probably want to use loft for this like you show, to get an even shape without scaling.

- Michael