I have a surface created from two 3D splines which share their end points (see attached sample). I can't figure out for the life of me why the surface is open at the point where the two splines meet.
Any help is greatly appreciated -- this has been driving me nuts!
Thanks
Uncheck - "Optimize surface" 
Thanks, but nope. No difference -- still an open surface.
If I understand you correctly.
The surface will always be open until you do something to close it, see image.
Hope this helps.
You are misunderstanding what is meant by open surface. If you sketch a circle and make a planar surface, you will get the same open surface message. An open surface is any surface that doesn't enclose a volume. It doesn't mean that there is a problem with your surface.
Jim S.
Ah, I see what you're getting at.
My original purpose for this surface is to use it to cut a solid, and I kept getting the dreaded "op failed due to geometric condition" message, so I then incorrectly assumed it was due to this.
Thanks for the clarification, I'll mark this one as answered and start a new discussion.
Darren,.. Yes, it is a open surface. I jumped to a conclusion that the surface was not manageable (it's has singularity) , that is, in this case, "unchecking" the fill allows for this surface be be "untrimmed" and easier to manage (see attached).....
You are misunderstanding what is meant by open surface. If you sketch a circle and make a planar surface, you will get the same open surface message. An open surface is any surface that doesn't enclose a volume. It doesn't mean that there is a problem with your surface.
Jim S.