A folding knife, commonly called a folder, is a knife whose blade folds into the handle when closed and pivots out into a straight position when open. The term "pocket knife" is often used interchangeably with "folder", but I prefer to think of pocket knives as a sub-type of folder, because "pocket knife" tends to call images of a red-handled Swiss Army Knife to mind, and there are folders that are smaller and larger than a knife expected to fit in a pocket would be.
For those of you who aren't familiar with knife-related terms, check out my writeup in the knife node, which has definitions for the terms I will use in this writeup.
While folding knives have existed at least since the Romans, the folding knife design has undergone significant changes recently and is still going through modifications.
Typically, a folder does not have a guard, as it would interfere with the folding action. Two exceptions are knives with extended finger grooves that form a makeshift guard, and knives with a flipper. Folders do not have tangs, either, as a tang is only needed to secure the blade in place into the handle for a fixed knife, which is not necessary for a folding knife.
Pretty much all folders are single-edged, as a double-edged blade would pose a hazard when the knife is closed and carried.
A folding knife has a few components not found in other kinds of knives, specifically a blade locking (or non-locking) mechanism and a method for opening and closing, often for opening the knife with one hand. Many folding knives (especially those with single blades, rather than multi-tools) will also have clips on the handle so that the knife can be clipped to the inside of the pocket.
(Note: Although the balisong, also known as the butterfly knife, is typically classified as a type of folding knife, the design is so unique that it is better served to be described in a separate writeup.)
Blade locking and non-locking mechanisms
Since folding knives' blades pivot in order to open and close, there is usually some system in place to prevent the blade from opening or closing when it shouldn't.
Non-locking folders: Non-locking folders are designed to pivot closed when a small amount of pressure is applied to the spine of the blade. Most knives with these mechanisms are made for light use, as these mechanisms aren't as strong or secure as locking mechanisms.
- Friction folder: This design uses friction between the blade and scales to hold the blade in place. Sometimes friction folders will have a catch to prevent the knife from opening when it is not wanted. Pretty much all friction folding knives must be opened with two hands. Friction folders aren't all that popular these days.
- Slipjoint: When the knife is open, the blade is held in place by a backspring that will create some resistance when there is pressure on the spine of the blade, but after that allows the blade to fold. The slipjoint is the typical mechanism that you'll find on the blade of a Swiss Army Knife, and is a very popular mechanism. Most slipjoint knives must be opened with two hands.
Locking mechanisms: A locking mechanism will hold a blade in place until a secondary action is used to disengage the lock. In other words, some lever or button needs to be pushed to let the blade close. Locking mechanisms generally make the knife safer, as chances of the blade accidentally closing on the user's fingers are reduced. The main types of locks are the lockback, linerlock, and framelock, although there are many less-popular locks, a few of which I've included.
- Lockback (back lock): This style of lock has a spring-loaded bar with a tooth at the end, which falls into a notch cut into the blade tang and is held there under the spring tension. A cut out in the handle spine holds the release for the lock. Traditionally, a lockback knife requires two hands to open and close, but there are a number of lockback knives that can be opened and closed with one hand. The lockback is one of the simplest locking mechanisms, and one of the sturdiest.
- Linerlock (locking liner): The actual locking mechanism is incorporated in the liner of the handle, hence the name. When the blade is opened, the liner, a metal sheet inside the handle, will bend and touch the base of the tang, locking it open. The lock is released by moving the liner to the side, an action that can be performed with the thumb on one hand. Generally, the linerlock is less secure than the lockback, but there have been modifications made to make it safer. The linerlock is one of the most popular locks on folders today, especially "tactical" folders. The linerlock was refined (but not invented) by knife maker Michael Walker.
- Framelock: The framelock is a variant of the linerlock, where the frame, instead of a liner, functions as an actual spring. It is usually much more secure than a linerlock.
- AXIS lock: The AXIS lock is rather complicated and beyond my abilities to explain, so it's probably best to use the explanation on the company's website. "AXIS gets its function from a small, hardened steel bar which rides forward and back in a slot machined into both steel liners. The bar extends to both sides of the knife, spanning the liners and positioned over the rear of the blade. It engages a ramped, tang portion of the knife blade when it is opened. Two omega style springs, one on each liner, give the locking bar it’s inertia to engage the knife tang, and as a result the tang is wedged solidly between a sizable stop pin and the AXIS bar itself." The AXIS lock is patented by Benchmade.
- Ringlock: The ringlock uses a rotating slipring that locks the knife in place.
- Block Lock: A spring-loaded block on a pin moves into a hole in the tang.
- Rolling Lock: A bearing rolls into a locked position.
Folders need to have some way to be opened. Some need to be opened with two hands, and some can be opened with one hand. There are also unassisted, assisted, and automatic opening mechanisms.
Unassisted opening, two-handed: Most older knives, multi-tools, and knives with multiple blades need to be opened two-handed.
- Nail nick: The traditional folding knife has a small slot for gripping with the index or middle finger's nail on the hand not holding the knife in order to open the knife up. This requires two hands to open the knife and can be slow and somewhat cumbersome. Swiss Army Knives are mostly nail nicks.
Unassisted opening, one-handed: The methods for one-handed unassisted opening knives are a recent invention, created in the late 80s and early 90s.
- Thumb stud: The stud is on the flat of the blade, where it that can be pushed on to push the blade forward, opening the knife one-handed.
- Thumb hole (Spyderhole): The hole is on the flat of the blade, where the user can put the pad of his thumb on it and push it to pivot the blade out. The thumb hole is patented by Spyderco. (Yes, you read that right. Spyderco patented a hole.)
- Flipper: The flipper is typically a small bump on the back of the blade that extends on the other side of the handle when the knife is closed. When it is pushed against with the index finger, the blade is pushed to pivot open. The flipper then becomes a small fingerguard.
- Wave: The Wave is a small hook facing outwards on the spine of the blade that catches the pocket lip, opening the blade as the knife is being pulled out of the pocket. On knives with thumb holes, it is also possible to make a "poor man's Wave" by attaching a lanyard to the hole or cutting the hole to make a Wave. A knife with a Wave is often called "waved blade" or "waved knife". The Wave is patented by Ernest Emerson.
- Sheath opening: There have been several sheath designs that have attempted to keep the blade temporarily in place while the handle is being pulled out, opening the knife in a similar manner to the Emerson Wave. I personally have not found a sheath that does this consistently, but there may be one that does, and if it doesn't exist yet, it's a matter of time until one is designed.
Assisted opening: An internal mechanism, typically an extra spring, is used to open the blade when slight pressure is applied to the thumbstud or flipper and the blade is opened partially. Assisted-opening mechanisms were designed largely to circumvent laws banning automatic opening knives.
Automatic opening: Most people call knives that have an automatic opening mechanism switchblades, automatic knives, or (in Britain) flick knives. These are either out the front or side-opening models. Some knives are also dual action, and can be opened both manually or automatically. Switchblades tend to be cheap, as they are often targeted towards mall ninjas, but there are a few high-quality switchblades. Also, the opening mechanism tends to make these knives less reliable and structurally sound than unassisted and assisted opening knives.
Folder can be carried with a few different methods.
- Clip: Many folding knives (with the notable exception of Swiss Army Knives), especially one-handed-opening knives and "tactical" folding knives, come with a clip on the flat of the handle that allows the knife to be clipped to the inside of a pocket, waistband, or other article of clothing. This allows for easy and quick accessibility, which, when combined with the one-handed opening methods described above, makes folders a viable option for self-defense knives. Clipping a folder on also protects the knife from bouncing around in a pocket where it would get scratched and dented by keys and loose change, as well as have the pivot mechanism fouled by pocket lint. Clips are usually metal or plastic and similar to the clips found on pens except thicker. Clips may be integral to the handle or removable, and some clips may be reversible (for different types of carry) or attachable on both sides of the handle (for lefties and righties). Clips allow the knife to be easily accessible, while keeping it lint-free and unscathed by pocket items such as coins. The clip system, labeled "Clip-it", was created by Sal Glesser, the founder of Spyderco.
- Sheath: There are a few folders designed to be carried in a sheath. Also, folders without clips and those with removable clips can have custom sheaths fitted.
- Lanyard hole: Some folders have a hole on the butt of the knife so that the knife can be attached to a lanyard or keychain.