The pack cloth material would then be 'hung' from within this frame. This was the only way to help distribute loads within the pack. External frame packs are very bulky and cumbersome to carry through crowds or on busses. They have too many exposed bars and straps and don't fit well on luggage racks on most trains. Needless to say, this is not the backpack of choice for most travelers, including myself.
Most modern backpacks have an internal frame.
Internal frame packs have of a number of flat aluminum rods that are strategically placed within the pack to help the pack keep its shape. These rods can be bent slightly to conform to the contours of the wearer's back as well. The overall shape of internal frame packs is tall and narrow, perfect for walking in crowds. The shape of the pack is generally maintained by what ever is in the pack. If it's half full, it can be cinched up and made even smaller.