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When you look in the mirror you see your face flipped left to right. What you don't see is your skin turned inside-out. That would be horrific! Now if we take the gorgeous Suzanne (Monkey) and tab into Edit Mode, and then enable the Normal direction indicators:

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we see all faces and vertices are facing outward:

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Now, if I chop off half of the mesh, set the Pivot Center to 3D Cursor:

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and select all A. Then duplicate shift+D and mirror ctrl+M along the X axis, the faces and vertices on the mirrored half are facing inward:

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This could be a programing error, or a bug, or maybe theres a very good reason for it. It can be quite irritating to have to keep flipping the Normals:

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I know that the Mirror Modifier doesn't have this problem, but what about ctrl+M, can anyone please enlighten me on this?

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  • $\begingroup$ Very interesting, in emulating this, I noticed that if you mirror on the combination of any two axes the normals don't flip. Tick all 3 and it flips again. -- Also, doing this in object mode preserves the normals. $\endgroup$ – iKlsR Jun 15 '13 at 23:42
  • $\begingroup$ somewhat related: blender.stackexchange.com/questions/1020/… $\endgroup$ – iKlsR Jun 15 '13 at 23:43
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The mirror tool (CTRLM) actually inverts the model by scaling it -1. This will flip the model, but will also invert the normals.

Here's what the docs say:

The mirror tool in Edit mode is similar to Mirroring in Object mode. It is exactly equivalent to scaling by -1 vertices, edges or faces around one chosen pivot point and in the direction of one chosen axis, only it is faster/handier.

I'm not exactly sure what the mirror modifier is doing, but I'm assuming it copies the vertices location along the axis and puts it in the negative location. Here's what the docs say:

The Mirror modifier automatically mirrors a mesh along its local X, Y and/or Z axes, which pass through the object’s center (the mirror plane is then defined by the two other axes).

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