Hi, im new to the forums :P, just started to use blender 4 or 5 days ago, now im trying to model a human body, but when i was checking the main body i found some weird problem between the torso and the pelvis…
The problem is only seen with smooth shading, with the subdivision surface can be seen better, but it is still unknown to me, hope these screenshots will help you as i cant detail the problem anymore:
The screenshots are these, not sure about the order (imageshack screwed it up):
-Rendered with subdivision surface (3) and smooth shading
-The same on the 3D Viewport
-3D Viewport, subdivision surface turned off, smooth shading
-3D Viewport, subdivision surface turned off, flat shading
First your topology in that area isn’t correct because you have triangles instead of quads. So my advice is: delete those faces and try to maintain always polygons with four vertices. (If you have the same trouble again and with quads, try to select all vertices and ‘remove doubles’ maybe you extruded two times one edge )
Basic checklist for these situations for me is usually to remove doubles and recalculate normals. Triangles can end up doing weird things, but I’m not sure that’s what’s going on here… it might help if you uploaded the blend.
I tried turning the triangles into quads, didnt help, the problem was still there, the same with the doubles, about the recalculate normal thing i think i did it correctly, though im not completely sure (searched for it in the space key menu, and tried with those options), here is the blend file as VPellen requested:
The problem is honestly the triangles. But it can be fixed. The first thing that you need to do is add an edgeloop on the side of the mesh going straight down from the armpit shooting down the side of the leg. Next is to delete the entire edgeloop that has nothing but triangles in it. After you’ve deleted all the triangles, create quads in their place. You can do this by clicking on 4 vertices, 2 at top 2 at bottom and clicking F to create face. Look at this.
Do you know what Ctrl-N does exactly?
Just to clear things out:
Every face has it’s front and back sides. The direction into which a face is looking is called “a normal”. You can view the Normal direction by going to Edit_mode/N/Display/Normals/Face. It will display it as a blue line.
Now, the shading gets messed up when two connected faces have a different Normal direction, that is, one points out of the model, the other points into the model. This mess is best visible with smooth shading.
Ctrl-N recalculates the normals of the selection, that is, makes all normals to point ouside of the model, which fixes the shading problem.
You can also do Ctrl-Shift-N to recalculate them to the inside, not the outside like Ctrl-N does.
Triangles are still bad in organic modeling though.
Here’s the reason why:
On the left the box had six faces.
On the right the box had a few triangulated faces.
If you subdivide smooth both of them, the one on the left looks well rounded while the one on the right looks like a mess.
(If you have the same trouble again and with quads, try to select all vertices and ‘remove doubles’ maybe you extruded two times one edge 
)
