The Packing Cube Shoulder Bag is a dual-purpose packing cube: in preparation for and during your journey, use it to organize and pack your clothing inside the Techonaut 30, Techonaut 45, Aeronaut 30, Aeronaut 45, Tri-Star, or Western Flyer — just like how you'd use our Packing Cubes. When you reach your destination and it's time to leave your main travel bag in the hotel or hostel, repack the Packing Cube Shoulder Bag with a light jacket, wallet, phone, small water bottle, map, and add a shoulder strap: you're ready to go out and explore, and you've saved space by not having to bring a separate bag for day use. Most folks who use our bags have at least one shoulder strap made by us, and thus we don't include a shoulder strap with the Packing Cube Shoulder Bag. If you would like a second shoulder strap for the Packing Cube Shoulder Bag, or simply a lighter shoulder strap, choose the optional 1" Shoulder Strap.