Gravity knife

A gravity knife is a knife with a blade contained in its handle, which opens its blade through 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 to allow opening and closing to be done with one hand, in situations where the other is occupied. Hence, historically, parachutists have been issued them to cut off caught lines, such as lines tangled in trees, a major potential use of the gravity knife. The gravity knife uses a button, trigger, or fulcrum lever to release the blade from both the open and the closed positions, and may use a side-folding or telescoping (out-the-front, or OTF) blade. While most military gravity knives utilize a locking blade design, other types may not mechanically lock open but rely instead upon friction to wedge the blade's rear section against the handle's interior. Factory-made gravity knives have various types of buttons, triggers, and fulcrum levers, which usually are used to release the blade from both the open and the closed positions.