Help understanding shading artifacts

[YAFU (English is not my native language)]

Hi.
Deselect all. Select menu > Select all by Trait > Non Manifold and you will see

Yep, one must face this problem sometime:

How to correctly model this meshes sharing edges, and Subsurf?

Hi. I’m surprised myself that I have not run into this problem before. How should I correctly model this ?: [image] http://www.pasteall.org/blend/39719 So when after applying Subdivision Surface Modifier and perhaps Solidify Modifier, make these imperfections do not appear. Is it possible to achieve this in these flat meshes using Crease edges or some other technique? Or I would have to model a mesh representing a volume from scratch (rather than a flat mesh)?