A knife bayonet is a knife which can be used both as a bayonet

Bayonet

A bayonet is a knife, dagger, sword, or spike-shaped weapon designed to fit in, on, over or underneath the muzzle of a rifle, musket or similar weapon, effectively turning the gun into a spear...

, fighting or utility knife

Knife

A knife is a cutting tool with an exposed cutting edge or blade, hand-held or otherwise, with or without a handle. Knives were used at least two-and-a-half million years ago, as evidenced by the Oldowan tools...

. The knife bayonet became the almost universal form of bayonet in the 20th century due to its versatility and effectiveness. The socket bayonets proved useless when separate from the rifle and useless in trench warfare

Trench warfare

Trench warfare is a form of occupied fighting lines, consisting largely of trenches, in which troops are largely immune to the enemy's small arms fire and are substantially sheltered from artillery...

; and while versatile, sword bayonet

Sword bayonet

thumb|300px|right|[[Ishapore 2A1]] Lee-Enfield w/ P1907 sword bayonetA sword bayonet is any long, knife-bladed bayonet designed for mounting on a musket or rifle. Its use is thought to have begun in the 18th century and to have reached its height of popularity throughout the 19th and into the early...

s proved to be impractical weapons in trench warfare because of their length.

The knife bayonets are basically fighting knives or utility knives with a lug and/or muzzle ring to attach to the barrel of a firearm such as an assault rifle

Assault rifle

An assault rifle is a selective fire rifle that uses an intermediate cartridge and a detachable magazine. Assault rifles are the standard infantry weapons in most modern armies...

 or submachine gun. Almost all bayonets today are knife bayonets that are designed for use in hand-to-hand fighting and as utility knives. Some have serrated (saw-blade) back edges for utility and be usable as wire cutters (when used in conjunction with fittings on the sheath

Sheath

Sheath may refer to:* Scabbard, a sheath for holding a sword, knife, or other large blade, as well as guns, such as rifles.* The outer covering of a cable* Condom, a kind of contraception* Debye sheath, a layer of a plasma in physics...

).
Modern knife bayonets have 6 inch to 9 inch long and .75 inch to 1.5 inch wide blades, and are often fuller

Fuller (weapon)

A fuller is a rounded or beveled groove or slot in the flat side of a blade . A fuller is often used to lighten the blade, much in the way that an I-beam shape allows a given amount of strength to be achieved with less material...

ed. They have been forged

Forging

Forging is a manufacturing process involving the shaping of metal using localized compressive forces. Forging is often classified according to the temperature at which it is performed: '"cold," "warm," or "hot" forging. Forged parts can range in weight from less than a kilogram to 580 metric tons...

for durability so as not to break when twisting; this was often the problem with long and slender socket bayonets. Like sword bayonets, they can be used not only for thrusting, but also for cutting.