Hi mir4ea,

> Another words, this surface can not be joined to its self?

When a surface is closed, it's basically the same thing as it being joined to itself - the closed area will be a "seam edge" that is a joined edge with the surface being joined to itself there.

You don't use the Join command to join a surface to itself, instead usually a closed surface is created by construction from a closed curve. Normally if you want a closed surface result you would start out with closed curves to construct it instead of starting with open curves to initially create the surface.


> It is just surface with ages perfectly aliened to each other?

Yes, that's what it means for a surface to be closed.


> And what happen with this surface edges when I put caps on both sided and join it it becomes solid?

Then you have a solid, it's just as much a solid as any other solid... A regular cylinder object is in fact made in exactly such a way - it's one closed surface for the round part and the top and bottom have trimmed planes for the end caps. You can't turn on control points for a solid like that since the different surfaces making it up do not share control points where they touch each other, since the caps are rectangular surfaces that have been trimmed. You can turn on control points again if you use Edit > Separate to break the solid apart into individual surfaces though.


> And how can I separate this aligned points?

There isn't any built in point editing tool for separating closed control points, just don't put them together in the first place if you don't want them together... If you really need to open it up again you can use Trim to cut away a small portion around the seam, that will then make an open surface again.

- Michael