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This has probably been asked, but I am not good with the modeling lingo yet so I have no Idea how to search this.

Why does a face on one side of the cube not go with the vertex? Its fine on the one side but on the other the face gets outside the edge. Face outside the edge

Can someone explain or link why this happens please.

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This is happening because of the way polygons are displayed. In order for the geometry to be displayed it needs to be triangulated as video cards draw only triangles behind the scenes(math is easier this way and pretty much all computer graphics currently are built on that). There are two ways to triangulate the side faces in your example:

enter image description here

What we are observing here is automatic triangulation algorithm failing to guess what kind of triangulation is desired in this case.

Errors like this will happen from time to time if you have concave n-gons. The best thing to do here is to get used to not having them in your geometry.

You can solve the issue manually connecting vertices of any concave geometry to make them convex as you can see in the GIF. If it happens that you need an automatic way to fix these issues you can also go to the Mesh menu in the header of any 3D View panel and choose Clean up -> Split Concave Faces while you have everything selected in edit mode:

enter image description here

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You have a 4 sided square that is folded almost in half, this is to be avoided when modelling.

What should be done instead when you start with the cube is to subdivide those bent faces into triangles first. Select the 2 vertex between where you want the square faces to bend then press J then repeat on the other side and there are the lines you are not getting.

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  • $\begingroup$ imho the question is asked because on the other side it's different, and is as expected... $\endgroup$ – m.ardito Nov 13 '17 at 8:16
  • $\begingroup$ and if you keep just the "good" face, and extrude it on Y, the same shape comes out different, ie: with looking good faces... $\endgroup$ – m.ardito Nov 13 '17 at 8:23
  • $\begingroup$ @m.ardito Got it. So the fix is very easy. But as m.ardito said I dont understand why it doesnt happen on both sides? $\endgroup$ – Gašper Skrt Nov 13 '17 at 9:53
  • $\begingroup$ @GašperSkrt You are doing something that is confusing to Blender so it won't display it properly. $\endgroup$ – Eric Huelin Nov 13 '17 at 12:19

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