Talk about functionality…. Not only can this bag be used as an expanding tank bag – it’s also designed to function as a tailpack. Wait, there’s more… Each section of the bag can be fully unzipped and used alone or reattached to different sections, in effect creating six bags in one. Only need a magnetic map bag? Just a tailpack with compression straps, no magnets? Just your map bag? Only a .25" tall magnetic mesh zipped pocket? It’s up to you. Zip off the top, middle and bottom and zip them in whatever configuration you choose. It’s a modular podular thing and it’s way cool. Top map bag section is padded and features a zipped, full-access 9.25"×12" window for oversize maps. Middle tailpack section has two zippered layers. The first features outer compression straps and inner oversize (9.25"×12.5") clear inner pockets with bookmarks. The second layer features a large zipped mesh pocket and a padded main compartment with 425 cubic inches of space that expands to over 700 cavernous cubic inches. The bottom section has four fold-away extra-magnetic feet and an oversize zipped mesh pocket. More? Two removable 7"×3"×1" side pockets stow sunglasses or whatever smaller items to make a total overall volume of about 900 cubic inches. Heavy duty nylon and fabric-reinforced vinyl construction. Shoulder strap, tailpack straps, and transparent-front rain cover included. Nice. 6"-9"×13.5"×3".