Join meshes : surfaces disappeared

I’m doing this tutorial, and I don’t understand why the surface added manually (8’25) disappeared when “join” is applied (11’22).
Thanks in advance for your answer.

Please categorize your posts.

Hello- keep watching - he explains that - the mesh normals are flipped inward on some faces. Rhino does not shade the back faces of closed meshes - unlike surfaces or polysurfaces, meshes can have inside out normals. It may be that UnifyMeshNormals would sort this out, but it is not necessary for what he’s doing.


@pascal indeed, he explains this , but I’m not sur that I well understood, thanks for these complementary explanations, it’s clear now.
@John_Brock sorry, but I don’t well understood how this works on the forum, is a tutorial or guide available for this ?

When you post a new question (create a new topic), it’s best to choose the category so it is seen by the right people, and so it can be more easily searched for later.
The only reason I mentioned it, is you have several posts in rapid succession and they were all uncategorized.


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Thanks for return.