Knife

thumb|upright=1.5|alt=Large knife with polished wooden handle, close to a leather sheath|A [[Bowie knife of pattern-welded steel]]

A knife (: knives; from Old Norse 'knife, dirk') is a tool or weapon with a cutting edge or blade, usually attached to a handle or hilt. One of the earliest tools used by humanity, knives appeared at least 2.5 million years ago, as evidenced by the Oldowan tools. Originally made of wood, bone, and stone (such as flint and obsidian), over the centuries, in step with improvements in both metallurgy and manufacturing, knife blades have been made from copper, bronze, iron, steel, ceramic, and titanium. Most modern knives have fixed or folding blades, with styles varying by maker and country.

Knives can serve various purposes. Hunters use the hunting knife, soldiers use the combat knife, scouts, campers, and hikers carry the pocketknife; there are kitchen knives for preparing foods (the chef's knife, the paring knife, bread knife, cleaver), table knife (butter knives and steak knives), weapons (daggers or switchblades), knives for throwing or juggling, and knives for religious ceremony or display (the kirpan).

Parts

right|thumb|alt=Different parts of a knife indicated with numerals|Characteristic parts of a knife

A modern knife consists of:

the blade
the handle
the point – the end of the knife used for piercing
the edge – the cutting surface of the knife extending from the point to the heel
the grind – the cross section shape of the blade
the spine – the thickest section of the blade; on a single-edged knife, the side opposite the edge; on a two-edged knife, more toward the middle
the fuller – a groove added to make the blade lighter (optional)
the ricasso – the flat section of the blade located at the junction of the blade and the knife's bolster or guard (optional)
the – the barrier between the blade and the handle which prevents the hand from slipping forward onto the blade and protects the hand from the external forces that are usually applied to the blade during use (optional)
the hilt or butt – the end of the handle used for blunt force
the lanyard – a strap used to secure the knife to the wrist (optional)

The blade edge can be plain or serrated, or a combination of both. Single-edged knives may have a reverse edge or false edge occupying a section of the spine. These edges are usually serrated and are used to further enhance function.

The handle, used to grip and manipulate the blade safely, may include a tang, a portion of the blade that extends into the handle. Knives are made with partial tangs (extending part way into the handle, known as "stick tangs") or full tangs (extending the full length of the handle, often visible on top and bottom). There is also the enterçado construction method present in antique knives from Brazil, such as the Sorocaban Knife, which consists in riveting a repurposed blade to the ricasso of a bladeless handle. The handle may include a bolster, a piece of heavy material (usually metal) situated at the front or rear of the handle. The bolster, as its name suggests, is used to mechanically strengthen the knife.

right|thumb|A [[Gerber Legendary Blades|Gerber-made full-tang survival knife. The metal from the blade extends into the handle.]]

Blade

thumb|Morakniv carbon steel knife

thumb|alt=Seated female workers making knives on machinery with large spinning drums|Knife blade mass production

Knife blades can be manufactured from a variety of materials, each of which has advantages and disadvantages. Carbon steel, an alloy of iron and carbon, can be very sharp. It holds its edge well, and remains easy to sharpen, but is vulnerable to rust and stains. Stainless steel is an alloy of iron, chromium, possibly nickel, and molybdenum, with only a small amount of carbon. It is not able to take quite as sharp an edge as carbon steel, but is highly resistant to corrosion. High carbon stainless steel is stainless steel with a higher amount of carbon, intended to incorporate the better attributes of carbon steel and stainless steel. High carbon stainless steel blades do not discolor or stain, and maintain a sharp edge. Laminated blades use multiple metals to create a layered structure, combining the attributes of both. For example, a harder, more brittle steel may be pressed between an outer layer of softer, tougher stainless steel to reduce vulnerability to corrosion. In this case, however, the part most affected by corrosion, the edge, is still vulnerable. Damascus steel is a form of pattern welding with similarities to laminate construction. Layers of different steel types are welded together, but then the stock is manipulated to create patterns in the steel.

Titanium is a metal that has a higher strength-to-weight ratio and more flexible than steel. Although less hard and unable to take as sharp an edge, carbides in the titanium alloy allow them to be heat-treated to a sufficient hardness. Ceramic blades are hard, brittle, lightweight, and do not corrode: they may maintain a sharp edge for years with no maintenance at all, but are fragile and will break if dropped on a hard surface or twisted in use. They can only be sharpened on silicon carbide sandpaper and appropriate grinding wheels. Plastic blades are not sharp and are usually serrated to enable them to cut. They are often disposable.

thumb|120px|left|Knife blades have different profiles

Steel blades are commonly shaped by forging or stock removal. Forged blades are made by heating a single piece of steel, then shaping the metal while hot using a hammer or press. Stock removal blades are shaped by grinding and removing metal. With both methods, after shaping, the steel must be heat treated. This involves heating the steel above its critical point, then quenching the blade to harden it. After hardening, the blade is tempered to remove stresses and make the blade tougher. Mass manufactured kitchen cutlery uses both the forging and stock removal processes. Forging tends to be reserved for manufacturers' more expensive product lines, and can often be distinguished from stock removal product lines by the presence of an integral bolster, though integral bolsters can be crafted through either shaping method.

Knives are sharpened in various ways. Flat ground blades have a profile that tapers from the thick spine to the sharp edge in a straight or convex line. Seen in cross section, the blade would form a long, thin triangle, or where the taper does not extend to the back of the blade, a long thin rectangle with one peaked side. Hollow ground blades have concave, beveled edges. The resulting blade has a thinner edge, so it may have better cutting ability for shallow cuts, but it is lighter and less durable than flat ground blades and will tend to bind in deep cuts. Serrated blade knives have a wavy, scalloped or saw-like blade. Serrated blades are more well suited for tasks that require aggressive 'sawing' motions, whereas plain edge blades are better suited for tasks that require push-through cuts (e.g., shaving, chopping, slicing).

Many knives have holes in the blade for various uses. Holes are commonly drilled in blades to reduce friction while cutting, increase single-handed usability of pocket knives, and, for butchers' knives, allow hanging out of the way when not in use.

Fixed-blade features

thumb|alt=Fixed-blade knife with wooden handle on a table, next to a molded plastic sheath|cKc pen knife

A fixed blade knife, sometimes called a sheath knife, does not fold or slide, and is typically stronger due to the tang, the extension of the blade into the handle, and lack of moving parts.

Folding blade features

thumb|alt=Folding pocket knife with multiple exposed tools|A [[Swiss Army knife]]

A folding knife connects the blade to the handle through a pivot, allowing the blade to fold into the handle. To prevent injury to the knife user through the blade accidentally closing on the user's hand, folding knives typically have a locking mechanism. Different locking mechanisms are favored by various individuals for reasons such as perceived strength (lock safety), legality, and ease of use.

Popular locking mechanisms include:

Slip joint – Found most commonly on traditional pocket knives, the opened blade does not lock, but is held in place by a spring device that allows the blade to fold if a certain amount of pressure is applied.
Lockback – Also known as the spine lock, the lockback includes a pivoted latch affixed to a spring, and can be disengaged only by pressing the latch down to release the blade.
Linerlock – Invented by Michael Walker, a Linerlock is a folding knife with a side-spring lock that can be opened and closed with one hand without repositioning the knife in the hand. The lock is self-adjusting for wear.
Compression Lock – A variant of the Liner Lock, it uses a small piece of metal at the tip of the lock to lock into a small corresponding impression in the blade. This creates a lock that does not disengage when the blade is torqued, instead of becoming more tightly locked. It is released by pressing the tab of metal to the side, to allow the blade to be placed into its groove set into the handle.
Collar lock – found on Opinel knives.
Button Lock – Found mainly on automatic knives, this type of lock uses a small push-button to open and release the knife.thumb|alt=Close-up of the pivot joint of a folding knife, showing locking barrel inserted through holes in the handle|The Benchmade Axis Lock mechanism
Axis Lock – A locking mechanism patented by Benchmade Knife Company until 2020. A cylindrical bearing is tensioned such that it will jump between the knife blade and some feature of the handle to lock the blade open.
Arc Lock – A locking mechanism exclusively licensed to SOG Specialty Knives. It differs from an axis lock in that the cylindrical bearing is tensioned by a rotary spring rather than an axial spring.
Ball Bearing Lock – A locking mechanism exclusively licensed to Spyderco. This lock is conceptually similar to the axis and arc locks but the bearing is instead a ball bearing.
Tri-Ad Lock – A locking mechanism exclusively licensed to Cold Steel. It is a form of lockback which incorporates a thick steel stop pin between the front of the latch and the back of the tang to transfer force from the blade into the handle.
PickLock – A round post on the back base of the blade locks into a hole in a spring tab in the handle. To close, manually lift (pick) the spring tab (lock) off the blade post with your fingers, or in "Italian Style Stilettos" swivel the bolster (hand guard) clockwise to lift the spring tab off the blade post.

Another prominent feature of many folding knives is the opening mechanism. Traditional pocket knives and Swiss Army knives commonly employ the nail nick, while modern folding knives more often use a stud, hole, disk, or flipper located on the blade, all of which have the benefit of allowing the user to open the knife with one hand.

The "wave" feature is another prominent design, which uses a part of the blade that protrudes outward to catch on one's pocket as it is drawn, thus opening the blade; this was patented by Ernest Emerson and is not only used on many of the Emerson knives, but also on knives produced by several other manufacturers, notably Spyderco and Cold Steel.

Automatic or switchblade knives open using the stored energy from a spring that is released when the user presses a button or lever or other actuator built into the handle of the knife. Automatic knives are severely restricted by law in the UK and most American states.

Increasingly common are assisted opening knives which use springs to propel the blade once the user has moved it past a certain angle. These differ from automatic or switchblade knives in that the blade is not released by means of a button or catch on the handle; rather, the blade itself is the actuator. Most assisted openers use flippers as their opening mechanism. Assisted opening knives can be as fast or faster than automatic knives to deploy.

Common locking mechanisms

framed|right

In the lock back, as in many folding knives, a stop pin acting on the top (or behind) the blade prevents it from rotating clockwise. A hook on the tang of the blade engages with a hook on the rocker bar which prevents the blade from rotating counter-clockwise. The rocker bar is held in position by a torsion bar. To release the knife the rocker bar is pushed downwards as indicated and pivots around the rocker pin, lifting the hook and freeing the blade.

When negative pressure (pushing down on the spine) is applied to the blade all the stress is transferred from the hook on the blade's tang to the hook on the rocker bar and thence to the small rocker pin. Excessive stress can shear one or both of these hooks rendering the knife effectively useless. Knife company Cold Steel uses a variant of the lock back called the Tri-Ad Lock which introduces a pin in front of the rocker bar to relieve stress on the rocker pin, has an elongated hole around the rocker pin to allow the mechanism to wear over time without losing strength and angles the hooks so that the faces no longer meet vertically.

The bolt in the bolt lock is a rectangle of metal that is constrained to slide only back and forward. When the knife is open a spring biases the bolt to the forward position where it rests above the tang of the blade preventing the blade from closing. Small knobs extend through the handle of the knife on both sides allowing the user to slide the bolt backward freeing the knife to close. The Axis Lock used by knife maker Benchmade is functionally identical to the bolt lock except that it uses a cylinder rather than a rectangle to trap the blade.

Plastic handles are more easily cared for than wooden handles, but can be slippery and become brittle over time.
Table knife or Case knife: A piece of cutlery, either a butter knife, steak knife, or both, that is part of a table setting, accompanying the fork and spoon

Tools

thumb|Western

right|thumb|Diver's knife from [[Three bolt equipment]]

thumb|Knives for cutting [[cheese]]

As a utility tool the knife can take many forms, including:

Bowie knife: Commonly, any large sheath knife, or a specific style of large knife popularized by Jim Bowie.

right|thumb|180px|A Head knife (Round knife).

Bushcraft knife: A sturdy, normally fixed blade knife used while camping in the wilderness.
Camping knife: A camping knife is used for camping and survival purposes in a wilderness environment.
Head knife or Round knife: A knife with a semicircular blade used since antiquity to cut leather.
Crooked knife: Sometimes referred to as a "curved knife", "carving knife" or in the Algonquian language the "mocotaugan" is a utilitarian knife used for carving.
Diver's knife: A knife adapted for use in diving and water sports and a necessary part of standard diving dress.
Electrician's knife: A short-bladed knife used to cut electrical insulation. Also, a folding knife with a large screw driver as well as a blade. Typically the screwdriver locks, but the blade may not lock.
Folding knife: A folding knife is a knife with one or more blades that fit inside the handle that can still fit in a pocket. It is also known as a jackknife or jack-knife.
Hunting knife: A knife used to dress large game.
Kiridashi: A small Japanese knife having a chisel grind and a sharp point, used as a general-purpose utility knife.
Linoleum knife: is a small knife that has a short, stiff blade with a curved point and a handle and is used to cut linoleum or other sheet materials.
Machete: A large heavy knife used to cut through thick vegetation such as sugar cane or jungle undergrowth; it may be used as an offensive weapon.
Marking knife: A woodworking tool used for marking out workpieces.
Palette knife: A knife, or frosting spatula, lacking a cutting edge, used by artists for tasks such as mixing and applying paint and in cooking for spreading icing.
Paper knife: Or a "letter opener" it is a knife made of metal or plastic, used for opening mail.
Pocketknife: a folding knife designed to be carried in a pants pocket. Subtypes include:
Lockback knife: a folding knife with a mechanism that locks the blade into the open position, preventing accidental closure while in use
Multi-tool and Swiss Army knife, which combine a folding knife blade with other tools and implements, such as pliers, scissors, or screwdrivers
Produce knife: A knife with a rectangular profile and a blunt front edge used by grocers to cut produce.
Rigging knife: A knife used to cut rigging in sailing vessels.
Scalpel: A medical knife, used to perform surgery.
Straight razor: A reusable knife blade used for shaving hair.
Survival knife: A sturdy knife, sometimes with a hollow handle filled with survival equipment.
Switchblade: A knife with a folding blade that springs out of the grip when a button or lever on the grip is pressed.
Utility knife: A short knife with a replaceable (typically) triangular blade, used for cutting sheet materials including card stock, paperboard, and corrugated fiberboard, also called a boxcutter knife or boxcutter
Wood carving knife and whittling knives: Knives used to shape wood in the arts of wood carving and whittling, often with short, thin replaceable blades for better control.
Craft knife: A scalpel-like form of non-retractable utility knife with a (typically) long handle and a replaceable pointed blade, used for precise, clean cutting in arts and crafts, often called an X-acto knife in the US and Canada after the popular brand name.

right|thumb|A simple letter opener, or paper knife

Traditional and religious implements

Athame: A typically black-handled and double-edged ritual knife used in Wicca and other derivative forms of Neopagan witchcraft. (see also Boline).
Dirk: A long bladed thrusting dagger worn by Scottish Highlanders for customary and ceremonial purposes.
Katar: An Indian push dagger sometimes used ceremonially.
Kilaya: A dagger used in Tibetan Buddhist rituals.
Kirpan: A ceremonial knife that all baptised Sikhs must wear as one of the five visible symbols of the Sikh faith (Kakars)
Kris: A dagger used in Indo-Malay cultures, often by nobility and sometimes in religious rituals
Kukri: A Nepalese knife used as a tool and weapon
Maguro bōchō: A traditional Japanese knife with a long specialized blade that is used to fillet large ocean fish.
Puukko: A traditional Finnish style woodcraft belt-knife used as a tool rather than a weapon
Seax: A Germanic single-edged knife, dagger or short sword used both as a tool and as a weapon.
Sgian-dubh: A small knife traditionally worn with the Highland and Isle dress (Kilt) of Scotland.
Ulu: An Inuit woman's all-purpose knife with a handle directly above a highly curved blade.
Yakutian knife: A traditional Yakuts knife used as a tool for wood carving and meat or fish cutting. Can be used as a part of yakutian ethnic costume.

Rituals and superstitions

thumb|250px|right|alt=Painting of an elderly man holding a young boy's head down with one hand; a winged angel restrains the man's other hand, which grasps a knife. A ram looks on from the side; in the background is a Renaissance landscape with hills, trees, and a castle.|[[Binding of Isaac|The Sacrifice of Isaac by Caravaggio, (1590–1610; Oil on canvas; Uffizi). Abraham is holding the sacrificial knife.]]

The knife plays a significant role in some cultures through ritual and superstition, as the knife was an essential tool for survival since early man. knives were included in some Anglo-Saxon burial rites, so the dead would not be defenseless in the next world. The knife plays an important role in some initiation rites, and many cultures perform rituals with a variety of knives, including the ceremonial sacrifices of animals. Samurai warriors, as part of bushido, could perform ritual suicide, or seppuku, with a tantō, a common Japanese knife. An athame, a ceremonial knife, is used in Wicca and derived forms of neopagan witchcraft.

In Greece, a black-handled knife placed under the pillow is used to keep away nightmares. As early as 1646 reference is made to a superstition of laying a knife across another piece of cutlery being a sign of witchcraft. A common belief is that if a knife is given as a gift, the relationship of the giver and recipient will be severed. Something such as a small coin, dove or a valuable item is exchanged for the gift, rendering "payment."

right|thumb|A man sharpening a knife in Nepal using a traditional method

Legislation

Some types of knives are restricted by law, and carrying of knives may be regulated, because they are often used in crime, although restrictions vary greatly by jurisdiction and type of knife. For example, some laws prohibit carrying knives in public while other laws prohibit possession of certain knives, such as switchblades.