Black Line

Hi!
I’m having a problem right now. I’m testing some tutorials and such, teaching how to do faces. Now I’ve made a decent face, then I was told to delete half of it, do the rest with a linked duplicate. the thing is, that when I’m going to join them and use the so-called “remove doubles” - function, a ugly black line shows up in the middle. Why is this, and how do I fix it?

Hi,

Some of your normals (the direction perpendicular to the face) are facing the opposite way from the rest, so Blender believes your mesh has a massive kink in it. Recalculate the normals outside the object by entering edit mode (TAB), selecting your entire model (A), pressing CTRL-N and accepting the popup. Your normals will be recalculated and your model will look correct again.

Thanks a lot! now I can finally complete my creation :D!!