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I am trying to understand loops. In the manual (https://docs.blender.org/manual/en/latest/modeling/meshes/structure.html#loops) I read:

edge loops. They connect vertices so that each one on the loop has exactly two neighbors that are not on the loop and placed on both sides of the loop (except the start and end vertex in case of poles).

I start with a cube and connect on it's top and bottom two vertices making a diagonal edge:

Cube with diagonals on top and bottom

I thought that in this mesh I should be able to get a loop selected containing the two diagonals and the vertical edges connecting them, since this would have at each vertex two edges on the loop and on both sides one edge that is not on the loop.

What am I getting wrong?

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  • $\begingroup$ The 2 corner vertices on the top face (bottom-left and top-right) are neighbours of more-than-1 vertex of the loop (the 2 other vertices of the top face). I think that explains why its not an edge loop. Each vertex should have its own neighbour. $\endgroup$ Commented Aug 13, 2024 at 8:04
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    $\begingroup$ @Lutzi That is not correct, a diagonal connection on top and bottom should create a loop that can be selected. I guess pwp made a mistake connecting those corner vertices. $\endgroup$ Commented Aug 13, 2024 at 8:45
  • $\begingroup$ @GordonBrinkmann I selected the vertices and pressed F to "fill". $\endgroup$ Commented Aug 13, 2024 at 8:47

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The question is how did you "connect" those two corner vertices at the top and bottom: with J or with F?

Beginners often use F, this is the New Edge/Face from Vertices tool. And this is exactly the problem, it only creates a new edge. The new edge only connects those two vertices and is not connected to the already existing face which the vertices are part of.

new edge tool

As you can see in the following screenshot after creating the new edge, when I select the top face it is still a single square face, only the boundary edges are selected, not the diagonal edge going across:

face and edge separate

You could also select the edge nopw and delete it without getting a hole at the top. What you have to do instead is use the shortcut J for the Connect Vertex Path tool. As the tooltip shows, this tool "creates edges, splitting faces".

connect vertex path

So after selecting two corner vertices and pressing J you can now select one of the two triangle faces at the top separately, and the new edge gets selected with it as well:

two split triangle faces

If you would delete this diagonal edge, you would have a hole at the top because now this edge is part of the two triangle faces. If you make diagonal edges with this tool, you can select an edge loop going around the cube diagonally.

edge loop select

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