They were designed to hook on the "rifleman's waist belt" of 1855...
Few troops received these rifleman's waist belts, using the infantry waist belt instead, which had not mountings or provision to mount the hooks of the knapsack straps. So the soldiers seem to have acted variously...
Hooking them together across the chest seems to have been common...
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Or might have hooked them under the waist belt, like these chaps... It was apparently understood that's what they were for, especially perhaps for parade, etc. (I'm told by reenactors this is very uncomfortable...)
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The 1866 Army Quartermaster photos has the soldier just looping the hooks under the waist belt...
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Many Union soldiers didn't bother with the knapsacks at all, discarding or storing them, and wearing a bedroll.