Basics

A knife is a sharp-edged hand tool used for cutting. A knife usually consists of a blade, usually less than 12 inches (30cm) in length, attached to a handle. 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.

The first knives were flint or other rock, chipped or ground to an edge, sometimes with a handle. Later on with advances in smelting and metallurgy the blades were made of bronze, iron, then steel. While the materials have changed over time, the basic design remains the same.

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 a handguard, so that fingers cannot slip onto the edge and be cut.

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 and, for its weight, stronger; on most 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.

Materials

Blades are usually made of steel(s), though there are a few knives using materials like high tech ceramic and titanium, but these are very uncommon. Stainless steels have gained popularity in the latter half of the twentieth century because they are highly resistant to corrosion (though they can rust under extreme conditions). There is a trade-off between edge-holding and resistance to corrosion, but newer semi-stainless steels like D2 may offer the best compromise. Modern stainless steels include S30V, 154CM, ATS-34, and 440C. Chromium is the major alloying element in stainless steels, it causes them to be 'stainless'.

High carbon steels, as their name implies are a high carbon, low chromium alloy, and are very prone to rust and pitting if not kept dry. They are generally used for their excellent 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. Forschner/Victorinox make decent cheap kitchen 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 a variety of knife blade shapes; some of the most common are listed below.
Blade types

A normal (1) blade has a curving edge, and flat back. A dull back lets the wielder use fingers to concentrate force; it also makes the knife heavier and stronger for its size. The curve concentrates force on a small point, making cutting easier. Therefore, the knife can chop as well as pick and slice.

A curved, 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 Double edged or spey (3) blade has two edges. The idea is to make a blade that cuts in either direction, with a strong sharp point. This shape is primarily used for fighting knives (daggers, bayonets) because it can cut in both directions, has a sharp point, and is strong.

A clip point (4) blade is like a normal blade with the tip "clipped" to make the tip thinner and sharper. The back edge of the clip can have a false edge that can be sharpened to make a second edge. The sharp tip makes the blade exceptional as a pick, or for cutting in tight places. If the false edge is sharpened it increases the knife's effectiveness in fighting. The Bowie has a clipped blade that is strong and good for fighting.

A sheepsfoot (5) knife has a straight edge, and a curved dull back. It gives the most control, because the dull back edge is made to be held by fingers. Sheepsfoot knives are good for whittling, including sheep's hooves.

A tanto style (6) knife is thick. It is superficially similar to the points on most Japanese long and short swords (katana and wakizashi), although is a bit of a misnomer as the traditional Japanese tanto (dagger) does not share this point style. 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. This sort of blade is 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. An example is a head knife, used in leatherworking both to scrape down leather (reducing thickness), and to make precise, rolling cuts to form shapes.

A drop-point blade is very similar to a clip point, but it features the back convexed down, rather than having a clip taken out of it. It handles much like the clip-point.