A gravity knife is a knife with a blade contained in its handle, and that opens its blade by the force of gravity. This mechanism of opening is fundamentally different from the switchblade, which extends its spring-propelled blade automatically upon the push of a button, switch, or fulcrum lever. The main purpose of gravity opening is that it allows opening and closing to be done one handed, in situations where the other hand is occupied. Hence, historically they have been issued to parachutists to cut off caught lines, such as lines tangled in trees, a major potential use of the gravity knife.
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