The folding differential of this Q-Bayo is 1.5" . One inch is standard for medium size packs. Below: the original Q-Bayo is still used for fitting occassional but we prefer the durability of the stainless rivet version above. These are at the base of the stays as opposed to the P&G Bayonets that are at the top of the frame stays. When the Q-Bayo is folded, the pack bag shortens and compresses to accommodate it. The compression/accordian effect takes place just below the Back Pad/Vent.