I'm trying to create a plane (via Planar command) out of these 4 lines, but nothing seems to happen. I have tried with both joined and unjoined lines.

Any advice?

Hi. The curves are not planar. You could easily make a surface by seperating them and running Network. Or select the edge of the surface and sweep it along one of those other curves to create a planar face...

jonah

Wow, that was embarassing. Thanks for the catch.

Like this ;)

Thanks Pilou. My real problem was lack of immersion into 3D. My 'top' and 'bottom' lines weren't parallel, and I had forgotten that 3, and only 3, lines define a plane.

Well, to be pendantic, there can be an arbitrary number of lines, as long as there are 3 non-parallel lines.

Marvelous geometry :)