The flat edge holds the binder contents. The D ring allows the sheets to lie flat in the binder along the straight edge. The Angle D ring is the same as a D ring except the straight edge is angled so the ring is wider at the bottom of the ring than at the top. This ring is especially good for larger capacity binders.
Tuffy rings are plastic rings.
The end of the ring hooks into the spine to close. The ring size of a binder is determined by the capacity of the ring, not the width of the binder spine. To determine the ring size of a round ring, measure the inside diameter. For a D ring and Angle Dring, measure the length of the straight or angled edge. If more than one color, the art needs to be color separated.