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How can I extrude a part of a surface, while leaving that part in place? That would mean the extrude function makes a copy.

For example, when I subdivide a face of a cube in 9 squares and extrude (E) the middle square, I get a little cube on top of the initial cube. However, when I hide the top face of the little cube it's clear that the little cube has no bottom. How can I extrude while leaving the bottom in place?

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The most practical procedure on ANY mesh would perhaps be select the Face, Shift+D (duplicate), cancel (Esc), extrude (E)

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  • $\begingroup$ Are all vertices connected? $\endgroup$ – atomicbezierslinger Dec 31 '15 at 16:29
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    $\begingroup$ who would have thought that such a hack would solve hours of work! I suggest deduplicating (merginge by distance) afterwards. And maybe reversing some normals. $\endgroup$ – arthur Apr 16 '20 at 21:12
  • $\begingroup$ Still works with Blender 2.83! @atomicbezierslinger Yes and no. It does create new edges and faces on the outside that connect the two "layers" but the inside vertices of the original layer aren't connected to any of the inside vertices of the new layer. $\endgroup$ – Neph Nov 3 '20 at 14:28
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You may use Edges Only option. With the Edge select enabled select the region you want to extrude, then press Ctrl+E-->Edges Only. You may then fill the gap with the F key. enter image description here

You may also just press E after you select the edges. After extruding the selected portion you may press Space and type Grid Fill to automatically fill the gap with mesh. enter image description here

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  • $\begingroup$ Reality unfortunately is not so simple. The surface I want to "extrude with bottom remaining" is a complex mesh, so it can't be recreated with "Fill" or even "Grid Fill" ... $\endgroup$ – TavernSenses Dec 22 '15 at 21:18
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    $\begingroup$ @TavernSenses it would help if you edit your question and show what specific "complex mesh" you are talking about! $\endgroup$ – user1853 Dec 23 '15 at 16:30
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    $\begingroup$ You can probably think yourself of a thousand meshes that can't be recreated with "Fill" or "Grid Fill", so what kind of mesh it is exactly does not matter for the argument. $\endgroup$ – TavernSenses Dec 26 '15 at 21:19
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Duplicate and Bridge Edge Loops

  • Select the Vertices you want.
  • [Shift D] Duplicate Move along some axis.

enter image description here

Result of Duplicate Move above with color for ease of viewing.

  • Select boundary of source and select boundary of destination. Vertex Groups may help you remember your selection. Menu Select Boundary Loop may assist.
  • Bridge Edge Loops
  • When you look closely you can see the hidden surfaces has identical faces as the duplicated surface

enter image description here

Image below is the original selected vertices that were duplicated.

enter image description here

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