A pocket knife is a type of folding knife with a blade that fits inside the handle. Blades are typically no larger than 3 to 5 in. (8 to 13 cm) in length, and are small enough that they may be carried in a pocket, hence the name. Pocket knives are very versatile tools, and may be used for anything from opening an envelope, to cutting twine, to slicing an apple.

Pocket knives can cost anywhere from $1 from gas stations and thrift stores, to thousands of dollars for custom, hand-crafted pieces. Most well-made knives start at around $20.

Slipjoint knives

A Swiss Army knife

Early pocket knives were usually slipjoints, meaning that the blade did not lock, but was held in place by a hammer and lever device that would fold if a certain amount of pressure was applied. These knives often had more than one blade, including an assortment of knife blade types (serrated, plain edged, saws) as well as a myriad of other tools such as cap lifters, corkscrews, and scissors. A large tool selection is the signature of the Swiss Army knife. These knives were produced mainly by Victorinox and Wenger and issued to the army and sold to the public.

Locking knives

The 1900s brought a new facet to the knife world with the popularization of locking pocket knives. Companies such as Buck, Benchmade, Spyderco , Gerber, Camillus, and Kershaw , to name a few, have created a wide range of products with locks of all types. The lockback knife (the most popular form) is similar to a slipjoint, except that instead of the hammer releasing the blade with pressure, the user has to press on a lever located on the back of the knife handle to release the blade, adding a level of safety. There are other types of locks; some of the more popular ones are the liner lock, the frame lock, and the Axis lock (A Benchmade patent). Even the Swiss Army knife product range has adopted the locks on some of their knives.

Other features

Traditional knives were opened using nail-nicks, or slots where the user's fingernail would enter to pull the blade out of the handle. This became somewhat cumbersome and required use of two hands, so there were innovations to remedy that. The thumbstud, a small stud on the blade that allows for one-handed opening, led the way for yet more innovations, such as the opening hole (a Spyderco patent where the user presses the pad of his thumb against a hole and opens the blade by rotating his thumb similar to the thumbstud), as well as the Emerson Wave system, where a hook catches the user's pocket upon removal and the blade is opened during a draw. One of the first one handed devices was the automatic spring release, also known as a switchblade.

Another innovation of Sal Glesser, Spyderco founder, was the clip system, which he named a "Clip-it". Clips are usually metal or plastic and similar to the clips found on pens except thicker. Clips allow the knife to be easy to accessible, while keeping it lint free and unscathed by pocket items such as change.

Legal issues

While nearly all pocket knives are legal to own, they are increasingly facing legal restrictions on their use. While pocket knives are almost always used as tools, they do have the potential to become weapons. In many places it is illegal to conceal knives larger than a certain size, or with certain locking or opening mechanisms. They are often banned or heavily restricted in secure areas, such as schools and airports. Switchblades and other "auto-openers" are banned entirely in many places, including 37 US states. Nevertheless, they retain a significant following, associated especially with the outdoors and those who practice camping, hunting, fishing, and so on. The Boy Scouts offer merit badges that involve knife work, such as Wood Carving.