A knife is a sharp-edged tool used for cutting and stabbing. The blade of a knife is usually pointed and may have one or two cutting edges. Knives have been used as weapons and tools since the Stone Age and were among the first tools used by man.

Stone age knives consist of flint or other rock, chipped or ground to an edge, sometimes with a handle, sometimes without. Later, the blades were made of bronze, iron, then steel. While the materials have evolved continuously, the basic design remains that of a blade attached to a handle by means of a tang (the part of the blade which is inserted into the handle).

Together with the fork and spoon, the knife has been a common eating utensil in the West since the Middle Ages. In the West, multi-purpose pocket knives and cooking knives are probably the most common knives today, as the importance of the knife as a weapon is declining.

Anatomy of a Knife

A knife consists of a blade, a tang and a handle. The tang is an extension of the blade into the handle. Some knives have quillions between the blade and the handle, so that fingers cannot slip onto the edge and be cut. Some knives also have a choil, i.e. a crack, finger-hole or other space between the edge and the handle, which improves the grip.

A blood groove or fuller is a groove up the side of a blade. According to a popular myth, it lets bleeding occur from an artery without removing the knife. In reality, its only function is to make larger knives and swords lighter; on smaller knives its function is purely decorative.

Some knives also have a shoulder in which the blade thickens as it meets the handle. This helps keep the knife from jamming in bone. In kitchen knives, it keeps chopped items from moving back toward the hand.

The handle of a large knife should be made of a non-slip material, such as Kraton, and should be thick enough that one's fingers just meet one's palm when the knife is gripped as tightly as possible. A hole in the end of the handles allows the knife to be hung or placed on a lanyard. A lanyard can be used to pull knives out (with a lever, if necessary) when they jam.

Blades

Materials

Blades are either stainless or high-carbon, and there's an inevitable trade-off between performance and resistance to stains and rust. Modern stainless steels include S30V, 154CM, ATS-34, 440C, though even they can be made to rust. Chromium is the major alloying element in stainless steels, it causes them to be 'stainless'.

A more stainless steel is H-1, which is low in carbon while high in Manganese, Nickel, and Sulphur, and contains Nitrogen as well. H-1 is almost impossible to rust in all but the most corrosive envronments. Carbon steels are as their name implies high in carbon, low chromium, are more prone to rust and pitting, and are generally developed with the goal of good edge-holding ability.

As of 2004 there are a variety of exotic steels and other materials used to form blades. Knife manufacturers such as Spyderco and Benchmade typically use 154CM, VG-10, S30V, and CPM440V, as well as several high-speed high-hardness tool steels like D2 and M2. Other manufacturers sometimes use titanium, stellite, talonite, and other cobalt-containing alloys. All three are more ductile than typical stainless steels, but have quite a vocal support group despite concerns about health effects of the latter two alloys' cobalt content. Damascus steel, which is layered and instantly recognizable by its beautiful patterns, is typically used in high-end knife blades and has respectable edge retention. There is typically more demand for stainless steels and exotic alloys in the utility, outdoor, and tactical knife categories than there is in the kitchen knife category.

Kitchen knife blades tend to fall into two categories. Some use stainless steels to prevent users from having to pamper their blades and to be more forgiving on those who put chefs' knives in the dishwasher. Others are high in carbon for edge-holding ability, the presumption being that kitchens are not the wilderness and that chefs are willing to clean knives properly in exchange for better edge retention in economy steels. Forschner/Victorinox makes decent cheap chefs' knives; higher-end manufacturers include Wustoff, Global, and Henckels. Some manufacturers, particularly of kitchen knives, make ceramic blades; these stay sharp longer but due to their hardness chip more readily, and an accidental drop may chip, crack, or shatter the blade.

Shapes

There are several basically different types of knife blades: normal, spey, clipped, sheeps-foot, tanto and ulu.

A normal (1) blade has a curving edge, and flat back. A dull back lets one use fingers to concentrate force, and makes the knife heavier and stronger for its size. The curve concentrates force, making cutting easier. Therefore, it can chop as well as pick and slice. The single edge is also less expensive to produce than a double edge. A curved or trailing-point (2) knife has a back edge that curves upward. This lets a lightweight knife have a larger curve on its edge. Such a knife is better for slicing than a normal knife. A spey (3) blade has two curved edges. The idea is to make a blade that slices in either direction, with a strong sharp point. This is the strongest traditional style of knife. It's used for fighting knives (dagger, switchblades, etc.) because it can cut both directions, has a point and is strong. Many persons believe that the best all-around blade is an asymmetric spey, with the larger curve on the lower side. This is called a dropped spey. A clipped (4) blade is like a normal blade with a clip off the tip to make the tip thinner and sharper. The back edge of the clip can have a "cut swedge" that can be sharpened to make second edge. The sharp tip makes the blade exceptional as a pick, or for cutting in tight places. If the clip is sharpened, this is an attempt to make a working knife double as a fighting knife. This is another favorite knife shape, although it is not as strong as a spey. The Bowie is an attempt to make a clipped blade that's good for fighting, and as strong as a spey. A sheepsfoot (5) knife has a straight edge, and a curved dull back. It give the most control, because the back, dull edge is made to be held by fingers. It's good for whittling, including sheep's hooves. A tanto (6) knife is thick, almost a bar. It was invented by the Japanese. The edge is straight. The point is actually a second edge on the end of the blade, swept back from the point at 80-60 degrees.

An ulu knife is a sharpened half-circle. It's all edge, with no point, and a handle in the middle. It's good for scraping, and sometimes chopping. It is the strongest knife-shape.

A drop-point blade is very similar to a clip point, but it features the black convexed down, rather than having a clip taken out of it. It's handing characteristics and overall form is very similar to the clip-point.

For whittling (artistic knife carving) a blade as short as 25mm (1 inch) is common.

Serrations on a blade help keep the blade sharp. The points protect the slicing areas from nicks. A good serration pattern will help a knife stay sharp three to ten times as long as a straight edge.

The edge is sharpened at different angles for different purposes. 15 to 25 degrees is a good all-around angle. Slicing knives should have sharper angles, down to ten degrees. Chopping knives need blunter angles, out to thirty degrees.

Knives can be categorized based on either form or function.

Form

Knives exist in several styles:

Fixed Blade Knives : A fixed blade is one whose blade and handle are formed by a single piece of metal. This type of blade is typically stronger and larger than a folding blade knife. Activities that require a durable and strong blade, such as hunting or warfare, typically rely on the fixed blade. Some famous fixed blade designs include the Ka-bar and Bowie knives.

Folding Knives : A folding knife is one that has a pivot between handle and blade, allowing the blade to fold into the handle. Most folding knives are relatively small working blades, pocket knives are nearly all folding knives.

Some folding knives have a locking mechanism:

• The most tranditional and commonplace lock is the slip-joint. This isn't really a lock at all, and is found most commonly on Swiss Army-style knives. It consists of a backspring that wedges itself into a notch on the tang on the back of the blade.
• The lockback is the simplest true locking knife. It is found on most low-end locking knives. It is like a slip-joint, but the lock consists of a latch rather than a backspring. To disengage, one presses the latch on the spine of the knife down, releasing the tang.
• The linerlock is found on most higher-end knives. This is more difficult to do than a lockback or a slipjoint, but when done properly, is secure and allows one to close a lock singlehandedly. It consists of a liner bent so that when the blade opens, the liner presses against the rear of the tang, preventing it from swinging back. To disengage, you press the liner to the side of the knife from where it is attached to the inside of the scales.
• The framelock is a variant of the linerlock, however, instead of using the liner, the frame functions as a actual spring. It is usually much more secure than a liner lock.
• There are many other modern locks with various degrees of effectiveness. Most of these are particular to single brands, most notably Benchmade's AXIS(tm) lock and SpyderCo's Compression(tm) lock.

Many folding knives (particularly locking models) have a small knob, or thumb-screw that allows the user to open the knife quickly with one hand.

Function

In general, knives are either working, or everyday use blades, or fighting knives. Some knives, such as the Scottish Dirk and Japanese Tanto function in both roles. Many knives are specific to a particular activity or occupation:

• A hunting knife is normally used to dress large game. It is often a normal, mild curve or a curved and clipped blade.
• A stockman's knife is a very versatile folding knife with three blades: a clip, a spey and a normal. It is one of the most popular folding knives ever made.
• Utility, or multi-tool knives may contain several blades, as well as other tools. Examples include Leatherman, SOG, Gerber and Victorinox (The "Swiss Army knife") tools.
• An electrician's knife is specially insulated to decrease the chance of shock.
• A kukri is a fighting knife with a deep forward curve.

• A machete is a long wide blade, used to chop through brush. This tool (larger than most knives, smaller than a sword) depends more on weight than a razor edge for it's cutting power.

• A survival knife is a sturdy knife, sometimes with a hollow handle filled with equipment. In the best hollow-handled knives, both blade and handle are cut from a single piece of steel. The end has an O-ring seal to keep water out of the handle. Often a small compass is set in the inside, protected part of the pommel/cap. The pommel may be adapted to pounding or chipping. Recommended fillings for the handle: a compass (usually in the pommel). Monofilament (for snares, fishing), 12 feet of black nylon thread and two needles, a couple of plastic ties, two barbed and one unbarbed fishhook (unbarbed doubles as a suture needle), butterfly bandages, halizone tablets, waterproof matches.
• Special purpose blades may not be made of metal. Plastic, glass, wood and ceramic knives exists. In most applications, these relatively fragile knives are used to avoid easy detection.

A sharp knife is a safe knife. Dull knives lead to excessive use of force to cut materials, increasing the chance that the blade may slip and the force will be transferred to an unintended destination such as yourself or another person or object. In Boy Scout parlance, an area within the radius of the arm and blade length combined is called the 'blood circle'. Also, a dull or damaged knife will inflict a worse wound than a relatively 'clean' cut from a sharp knife.

Knives proffered to another person are always offered handle first.

A knife should be kept clean, dry and sharp. Steel blades rust easily, but oiling will prevent pitting due to oxidation and tarnish. Most knives are not intended as pry bars or screwdrivers. Either use is likely to break off the tip of the blade, or to bend or break the knife horribly. Most high quality knives are also tempered very hard, so that they will retain an edge longer. However, this also makes them brittle.

Knives are sharpened by grinding against a hard surface, typically stone. The smaller the angle between the blade and stone, the sharper the knife will be, but the faster it will dull. A guide is very helpful. Very sharp knives sharpen at 12 degrees. Typical knives sharpen at 22 degrees. Knives that chop may sharpen at 25 degrees. In short, the harder the material to be cut the higher the angle of the edge. The composition of the stone affects the sharpness of the blade (finer grain produces sharper blades), as does the composition of the blade (some metals take/keep an edge better than others).

Examples of sharpening tools are the clamp-style systems, which use a clamp with several holes with pre-defined angles. The stone is mounted on a rod and is pulled through these holes, so that the angle remains consistent. Another variant is the crock stick setup, where two sticks are put into a plastic or wooden base to form a V shape. When you pull a knife up the V, the angle is held for you, as long as you hold the blade perpendicular to the base.

Remove a wire edge (burr) if one forms during sharpening. Use a slighly steeper angle with very light pressure to do so. If not removed, it will break off in use, and the knife will instantly become dull. An alternate method of removing a wire edge is stroking from side to side on a very fine stone, using light strokes. This will flip the burr back and forth as it is ground off.

To feel for a wire edge, drag your thumb or thumbnail from spine to edge. It should come off with no resistance. If you feel a little bit of pull at the edge or the nail is sightly abraided, you may have a wire burr.

Traditional stones are arkansas stones, which come in soft (coarse) and hard (fine) varieties. These are traditionally used with honing oil, but any oil compound, including olive oil, will do. India stones are also similar to arkansas stones in function and end result.

Ceramic stones are also use very often. Unlike natural stones, they will not wear, and are very often used without any type of oil.

Water stones (both artificial and natural) come in very fine grits. They are stored in water, and develop a layer of slurry which helps polish off the edge. Generally, these are more costly than oilstones. Needless to say, oil should not be used on these.

Oil is used to lift the metal dust, called swarf, off the stone. This is usually recommended for natural stones, but some say that detergent and a brillo pad will have the same effect. Ceramic stones do not need oil, they can be cleaned off with detergent. Diamond stones should NOT be used with oil, rather, they can be washed off with water.

The best sharpening stones are industrial diamonds embedded in plastic or metal. These are more expensive, but still affordable. They cut about twice as fast as other stones. Also popular, and rather expensive, are sharpening blocks made from corundum.

Stropping a knife is an excellent finishing step. This is traditionally done with a leather strap impregnated with abrasive compounds, but can be done on paper, cardstock, or even cloth in a pinch. It will not cut the edge significantly, but produces a very sharp edge with very little metal loss. It is useful when a knife is still sharp, but has lost that 'scary sharp' edge from use.

Steeling isn't actually sharpening nor honing. All you are doing with a sharpening steel is realigning the edge. Realigning the edge goes a long way in keeping the knife sharp, as often times, a rolled edge will make an otherwise sharp knife dull. There is no reason to remove excess metal if the edge can be fixed with several strokes on a sharpening steel.

Even small knives are forbidden on all commercial airlines and are among the illegal imports that may be confiscated at airports by customs staff even if packed in luggage. The knife laws of different countries vary, but are generally strict in Western countries.

Knife laws in U.S. cities vary tremendously. In Texas, for example, individuals may carry knives openly or concealed so long as they are single-edged, and are not daggers, switchblades, or gravity knives (balisong legality is questionable — there have been convictions). In some other States, fixed-blade knives are banned, open carry is banned, and sometimes concealed carry of anything except pocketknives is banned. Cities have ordinances further restricting these laws; in San Antonio, TX, it is a violation to carry a folding knife having a locking blade. In some metro areas such as Washington, D.C., going into office buildings or museums, or simply loitering, carrying even small 3" folding knives can be problematic. Other restricted areas in the U.S. include court buildings and federal property (the latter of which technically has a limit of 2" for blades).

Carrying knives in public is generally forbidden by law in many countries. Exceptions may be made for hunting knives, and for knives used for work-related purposes (e.g. chef's knives). Automatic knives (switchblades) are almost universally banned from civilian carry if not possession. Balisongs (butterfly knives) are only slightly less stigmatized, and tend to be treated as switchblades by unfriendly law enforcement agencies. Most Western European nations are very unfriendly toward all knives other than Swiss Army Knives, by Victorinox or Wenger and similarly small tools, which are nonetheless not allowed on planes or in certain other venues. Even multitools like the SwissTool, Gerber multitools, and Leatherman multitools are often frowned upon, due to their having relatively large blades and/or locking ability.

Knives can be customized to the user and/or application:

• The handle can be altered in shape (for better grip) or material (to prevent shock, burns and freezing).
• The surface finish of the blade can be darkened or polished (to decrease visibility, or act as a signalling mirror, respectively).

• The Knife Bible, by Don Paul

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"Knives" is also an episode of the science-fiction television series Babylon 5; see Knives (Babylon 5).

As a verb, to knife is to use a knife to injure or kill by stabbing, slashing, or otherwise using the sharp edge of the knife as a weapon.