Tailored to fit the Harmony Blue Barrels. Can be used as pack without a Barrel. Store your water or fuel on the outside of the pack. Zippered top pocket for keeping small items handy. Top pocket on the 30 Liter is 4" tall with an 11" Diameter. 2#8 YKK zipper pulls behind zipper flaps allow easy access into two ample sized side pockets; 4"x 4" x 10" . Top pocket flap is held down with two Fastex quick release buckles.