A gravity knife is a knife which opens solely by the forces of inertia or gravity. Similar to a switchblae, they are made as side folding blades and OTF blades (out-the-front or telescoping). But instead of being spring driven, the knife is "flipped" out of the handle. Military gravity knives will lock open. Some English and civilian gravity knives do not mechanically lock open, instead containing a strong friction cam surface against the blade. Factory made gravity knives have various types of buttons, triggers or fulcrum levers, but are usually dual function to release the blade from both the open and closed positions.