A "crease" is where polygons come together. A corner, as it were. "smoothing groups" are creases. They can be classed as peaks or valleys for A "dart" is where a crease "ends" in the middle of a flat surface. Kind of hard to describe. The vertex that terminates the dart has shared normals for the smooth side, and individual normals for the crease side. And, of course, there's a "edge" where the mesh ends, and you fall off the edge of the world. Look at Hoppe's web site. The subdivision surface paper has nice illustrations. Ciao! Joe