"Gravity knife" means any knife that has a [blade] released from the handle or [sheath] thereof by the force of gravity or the application of [centrifugal force], that when released is locked into place by means of a button, spring, lever or other device.

[Colorado] [Criminal Code]: Section 18-12-101

Gravity knives are a rather neat type of knife, that are unfortunately illegal in many places. While the legal definition above could include many sorts of knives with pivoting blades, such as [butterfly knives], a true gravity knife has a blade that slides out of the handle.

The blade runs along a track down the center of the handle. While not in use it is prevented from slipping out by a catch. By holding down a [catch] release the knife can be "unsheathed" with a [flick of the wrist], or simply by pointing the blade opening downwards. Letting go of the release button locks the knife out. While there is no [spring] to propell the knife out of its handle, there is one to make sure that the catch is forced into the locked position while not held down. It is this [spring] that is the downfall of most [old] [gravity] [knive]s.

Gravity knives are uncommon, and the only notable example of their use in a military capacity was by [German] [paratrooper]s in [WW2]. These knives had wooden handles, and were equipped with a thin spike, like many sailing knives are today for the purpose of untying [knot]s. They had a [blade] that was sharpened along one edge only, and of about 4 [inch]es in length.

gravity knife (thing)