Neck Knife? it actually exactly what you need. Camping and you are trying to tie up a tarp, its on your chest, around your neck with its chain. You just reach down, give it a pull and your cutting that rope in seconds. Snap....its back in the sheath.
This one is D2 steel, super sturdy, stays sharp longer and a great sheath.
O/A length 5", 2.5" blade
Check out the video below, different knife, but same idea.
