a bag that sits below the seat, inside the rear triangle. i have one for my soma, but it's jury rigged. rear triangle bags used to be manufactured. fits a tube and a little repair items, or a trangia and assorted small stuff.

i like climbers chalk bags, strung off the saddle, used them for years to haul bananas and gu packs. Those Jannds are a great price. I simply sew a loop of webbing on the bottoms, and string 2mm cord to the fork, tie the chalk bag loops to the stem and handlebar, tighten the cords, and voila - feedbag. works like a charm. the revelate design feedbags are engineered for food, though - they are quite dope.