I have only been using Blender for a few days, so please go easy on me.
I'm working on a face, and I have subdivision surface visible while I'm editing. I have only ever used quads, but strange lines are visible on the mesh when viewed in object mode, and they turn into pinches when I add more levels of subdivision surface.
Sometimes I have to add another line running halfway or a quarter the way across the face, because I find that I don't have enough vertices to create only quads. When I do this, the line is visible in object mode as well, and changing to wire-frame shows that the subdivisions next to this new line turn inwards and cross at every vertex on the new line. This creates a line of thin triangle-like diamond shapes.
I can easily solve this by deleting the new line and creating it with extrusions instead, but this calls into question the point of using subdivide in the first place, and requires more tweaking afterwards. Could someone please explain why it does this, or tell me how to use subdivide without fighting these diamonds?
EDIT: It was something as simple as deleting the face before subdividing out the extra line, because Blender wouldn't separate a face into two even if there was a line running through it. Looks like "Loop Cut And Slide" does the face conversion automatically. Sorry for wasting your time, please delete my question.