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(two planes with merged vertices)

enter image description here

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It's a normals orientation issue. First, check your normals by enabling the Normals Overlay:

check normals

  • blue normals are pointing outside
  • red normals are pointing inside

The shading error comes from Blender not knowing how to get from one orientation to the other any more. Fix the normals by re-calculating them (this will make the orientation consistent across the model) in Edit Mode:

fix normals

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  • $\begingroup$ Thank you this worked! Very helpful. Thank you for the explanation, makes me understand blender more. :) $\endgroup$ – Marc Andrew Picardal Aug 16 '19 at 22:35
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Select all of the mesh A in edit mode and press CtrlShiftN. This will recalculate the normals.

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    $\begingroup$ What does that do and how does it help? And Select all the mesh means selecting the vertices or edges or faces or the objects transform? $\endgroup$ – Leander Aug 16 '19 at 7:21
  • $\begingroup$ @Leander it recalculates normals, select all the mesh in edit mode, doesn't matter if you're in edge select mode or face or vertex, just press "a" in edit mode to select all the mesh then press ctrl+shift+n. $\endgroup$ – Mohammadreza Souzandeh Aug 16 '19 at 8:23
  • $\begingroup$ @Leander also as aliasguru explained check the orientation of the normals, you can use shift+n instead of ctrl+shift+n based on which direction you need. $\endgroup$ – Mohammadreza Souzandeh Aug 16 '19 at 8:28

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