The Dual is so named because you can fit two 700c tubes, two gas cartridges and a gas valve, plus have room left over. Alternately, it will fit an ATB tube and gas, no problem. The leather patch prevents the seat post clamp from wearing a hole in the tubes inside. The simple Velcro strap is sewn double thick to hold its shape and to keep the bag cinched tight. 27 cubic in.