Plane Problem

 From:  Michael Gibson
2884.2 In reply to 2884.1 
Hi NightCabbage, yup that is normal - what you see there is the "underlying surface" which is a plane that is made up of 4 control points.

All NURBS surfaces have an underlying surface, which has a rectangular type grid of points. (not necessarily just 4 points, on other surfaces especially curved ones, the point grid can have more points in it than that, the 4 point one is the most basic one).

Then there can be trim curves on that surface to cut different areas away or make holes.

When you use Construct / Planar, it builds a large plane going through those curves, and then those curves become trim curves on that plane.

There are some other illustrations showing that exact same kind of thing in that FAQ answer

That whole mechanism of "underlying surfaces" and "trim curves" are one of the big differences with NURBS models as opposed to polygon models.

- Michael