A slightly clueless discussion of Blender 3d Quads and Topology by DOLLPARTZ
Let's start with a simple plane. At this point, if you do a loop cut you only get a straight line. So we introduce the first loop manually, so to speak, like so with an exact cut..
No problems there, all quads and it's all primed for loop cuts. All primed? Yes, cuz next if you do a loop cut now you find it will pick a circle around the first circle if you position the mouse in the right spot (see C). It will not work inside the first circle, only outside it, and only within the outer box (marked blue). You can really pack in the loops if yer so inclined, for, say, muscle bulges or folds in clothing. More about this in a minute.
Now one thing about the circle we made in B: There are some INTERNAL quads not like the others. I'm just talking about the quads on the inside and not on the edges of the plane, where quads can have even vertices with only 2 edges at the corners.
Four internal quads have a vertex that has only 3 edges, and that vertex is shown as orange in the diagram. These quads can do interesting things ... One thing they can do is reduce polys by taking three incoming edges from one said and reduce them to one on the other side. If you subsurf a mesh that has triangles in it, the triangles disappear, and what you will see are these very quads as shown in Figure D. I assume the modeller HAD triangles in the more primitive mesh, but after subsurf, the triangles were gone and we see these 3 poles as the quads are very similar and obviously made by subsurfing ...
If I wanna show muscles slanting this way and that, they are gonna show up these triangles. The narrowing of the arm to the wrist is a typical place where triangles happen. But I know that subsurf will cause them to disppear later, so why worry too much? Well, one thing, they block edge loops. I can always remove them by hand somehow, some way. I don't think I should be too concerned about triangles at least during the initial laying out of the mesh when I'm more concerned about following the flow of the muscles.
Besides, quads are formed by two triangles, and I read that the render process breaks everything down to triangles anyway.
*You will see many or most tutorials on the web create human figures without alot of musculature, and so the mesh is quite simple. But if you plan on showing muscles and bones showing under the skin (Figure E), eventually you realize that it's a whole new ballgame, you have to follow these lines as you first build the mesh, cuz they spread out, they narrow, one bulge will even pass over another bulge. So you must follow the lines of the surface. Otherwise it will be hell to put them in later. To do this I extrude edges rather than faces.
Blender will usually refuse to create concave quads when filling in a face but not always. There's a potential concave quad in figure F where the bicep of an arm meets the forearm. In this case, if I select the vertices and hit F, Blender will fill it. Other times it will not. And they can be created sneakily by pushing the vertex inward from a regular existing quad and Blender will not protest. A concave quad is when one of the vertices is inward, forming a dent. Concave quads should be avoided as two triangles are overlapping in the same plane. The easiest way to deal with them is to make them convex. In F this can't be done cuz of the curve shape of the bicep, so I remade it into a quad and a triangle.
More to come