Hi Danny, all NURBS surfaces have an implicit starting and ending location, but some CAD systems like ones based on Parasolid do allow trimming boundaries to cross over that parametric seam as if it weren't there and so the seam might not be represented in their 3D edge topology.

That makes for a fairly more complex type of internal trimming structure than one where each edge maps to one single curve in the UV parameter space though.

But that sort of does move the seam to something that's treated more by internal program logic rather than directly represented in the visible topology.

- Michael