We all want to avoid removing and putting on the pack to get certain things when backpacking, but don't like the usual solutions such as a string of small pockets suspended from our belt and banging our legs, or from shoulder straps that get all tangled up.  This shoulder bag is the right size for 1-quart canteens, a camera, bear-spray, a GPS, radio, compass, map, etc.  If you put it on after the pack and hang the strap behind your neck and one arm, it will ride comfortably in front and provide some balance to the weight in back, in addition to the convenience.  I don't hit the trail, even on day-hikes, without it anymore.