A knife is a handheld sharp-edged instrument consisting of handle attached to a blade used for cutting. The knife is a tool that can be used as a weapon. Its origins date as far back as two-and-a-half million years ago.
The earliest knives were shaped by striking rocks against each other, particularly harder rocks such as obsidian and flint. As recent as five thousand years ago, as advances in metallurgy progressed, stone, wood, and bone blades were gradually succeeded by copper, bronze, iron, and eventually steel. Modern knives may be made from many different materials such as alloy tool steels, carbon fiber, ceramics, and titanium. There is a very active community of modern custom knife makers and collectors, whom often pioneer the use of new materials in knives.
Today, knives come in many forms but can be generally categorized between two broad types: fixed blade knives and folding, or pocket, knives.
A fixed blade 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 movable parts.
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.
Modern knives consist of a blade and handle. The blade edge can be plain or serrated or a combination of both. The handle, used to grip and manipulate the blade safely, may include the tang, a portion of the blade that extends into the handle. The handle can also include a bolster, which is a piece of material used to balance the knife, usually brass or other metal, at the front of the handle where it meets the blade. The blade consists of 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 top, thicker portion of the blade, the fuller, the groove added to lighten and stiffen the blade, and the ricasso, the thick portion of the blade joining the blade and the handle. The guard is a barrier between the blade and the handle which protects the hand from an opponent, or the blade of the knife itself. A choil, where the blade is unsharpened and possibly indented as it meets the handle, may be used to prevent scratches to the handle when sharpening or as a forward-finger grip. The end of the handle, or butt may have a lanyard used to secure the knife to the wrist.
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, hold its edge well, and remain 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 combine the best attributes of carbon steel and stainless steel. High carbon stainless steel blades do not discolor or stain, and maintain a sharp edge. Laminate blades use multiple metals to create a layered sandwich, combining the attributes of both. For example, a harder, more brittle steel may be sandwiched 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. Pattern-welding is similar 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 metal that is lighter, more wear resistant, 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 incredibly hard, lightweight blades; so hard that they will maintain a sharp edge for months or years with no maintenance at all. They are immune to corrosion, but can only be sharpened on silicon carbide sandpaper and some grinding wheels.
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. Forging tends to be reserved for manufacturers' more expensive product lines.