Attachment system. Surprised me that it came with a rain cover for your saddle too

Not a whole lot of room, but enough.

Best feature of this bag is the attachment system. It has a separate clamp that attaches to seat rails and then the seat 'screws' on with a quarter turn. Very secure and stable. Best of all, there's no velcro strap to rub the decal off my Keith Eriksson seat post!