Hi, there’s this editing problem I have never encountered before.
Right now I’m doing a little bit Face Modlling and I started by defining the Mesh for the basic Face Parts, just like in the “Model the female body” tutorial on blendercookie.
When I try to subdivide those loop-Meshes, I get ugly cracks at some edges. Note that not every edge is affected, but quite a lot though.
This is the Mesh in Object Mode using Smooth Shading
Doubles are duplicated vertices. If you duplicate or cancel an extrude it duplicates your vertices by they are in exactly the same positions as the originals so you can’t see them. This can cause lots of problems so before trying to fix any issues like yours the first thing is to remove them just in case there are any. When you remove doubles in the toolshelf (keypad T or F6) there is a slider that sets how close vertices should be to be classed as a double and hence merged with other vertices within that distance. Generally you don’t need to change this threshold.
The real cause of your problem is that you have some faces pointing forward and some faces pointing backwards. In the Properties (N) panel undr dispaly in edir mode you can select show normals so you can see which way the face is pointing. Ctrl+N recalculates the face directions to be consistent (this can be fooled if you have doubles vertices, hence removing any before recalculating). Subdividing faces can fause this issue, its always good proactice to recalculate if you see this ocurr as it can then cause problems when you use operations that rely on the face normal direction (such as the solidify modifier)
