Glossary
D-ring binder
Also called: D ring vs O ring, round ring binder, O-ring binder
A D-ring binder has straight-edged rings mounted flat to the back cover, so pages stack against a flat wall and lie flat; round (O) rings force pages into a curve, which Avery says can bend the cards nearest the rings.
Avery's guidance for trading card binders states that pages should rest as flat as possible in storage so the cards near the rings don't flex, and that round rings "can create a curve that can bend your cards" as well as making pages harder to turn in groups.
The reason is geometry. A round ring is a circle, so a stack of pages threaded onto it has to bend around the curve. A D-ring presents a straight edge for the pages to sit against, and because it is mounted to the back cover rather than the spine, the stack sits flat when the binder is closed.
There is a capacity difference too. Avery rates its D-ring binders at up to 50% more sheets than a round-ring binder with the same spine width, because the D shape uses the space more efficiently than a circle.
None of this makes a D-ring binder the best option for cards, only the better of the two ring types. Binders with no rings at all, meaning zipper and strap closures holding a fixed block of pages, remove the question entirely. Most collector-focused binders are now built that way.