Its evolution can be traced to a certain extent to a fortuitous accident. In the mid seventeenth century irregular conflicts of rural France, the peasants of the Southern French town of Bayonne, having run out of powder and shot, rammed their long bladed hunting knives into the muzzles of their primitive muskets to fashion impromptu spears, and by necessity created an ancillary weapon that was to influence Western European infantry tactics until the early 20th century.
The benefit of such a dual-purpose arm contained in one was soon apparent. The early muskets fired at a slow rate (about a round a minute when loading with loose powder and ball), and were unreliable. Bayonets provided a useful addition to the weapon-system when an enemy charging to contact could cross the musket's killing ground (a range of approx 100 yardss/metres at the most optimistic) at the expense of perhaps only one volley from their waiting opponents. A foot long bayonet (extending to a regulation 17 incheses (approx 43 centimetres) during the Napoleonic period, on a 6 foot (almost 2 meter) tall musket achieved a reach similar to the infantry spear, and later halberd, of earlier times.
Early bayonets were of the "plug" type. The bayonet had a round handle that fit directly into the musket barrel. This naturally prevented the gun from being fired.
Later "socket" bayonets offset the blade from the muzzle. The bayonet attached over the outside of the barrel with a ring-shaped socket, secured on later models by a spring -loaded catch on the muzzle of the musket barrel.
Many socket bayonets were triangular in order to privide sideways stability of the blade without much increase in weight. This design of bayonet did not include a handle to use the blade apart from the gun.
18th century and 19th century military tactics included various massed bayonet charges and defenses. The British Army was particulary known for its bayonet use, although towards the early 19th century and the flowering of Napoleonic warfare, the primacy of regular and speedy volley-fire saw the British eclipse their opponents in line to line infantry combat.
There are rumours among old (pre World War I) soldiers of exotic bayonet techniques, almost as complex and involved as sword-fighting. Supposedly, rather than just the modern simplified blocks and thrusts, there were also cuts, counters and disarms, in which a sliding block would lead to an attack or disarmament. Supposedly, these techniques also taught use of edge and point, and special vulnerabilities such as wrists, ankles, neck, brachial and femoral arteries. Further, all types of moves are said to have been practiced in every orientation, and relative position of the two fighters' weapons, in training methods similar to advanced sword-fighting. These techniques were possible because of the long periods of continued training of the professional armies before this period.
A late 19th century Prussian bayonet. A public domain image.
In the Geneva Accords on Humane Warfare, triangular and cross-sectional bayonets were outlawed because the wounds they produce do not close easily, and were said to be inhumane. Despite these limitations, all modern bayonets have a blood groove (visible on the top half of the blade shown above), which is a concave depression in the blade designed to prevent the elastic properties of the human body from thwarting a successful bayonet thrust. Such a groove also facilitates removal of the bayonet after insertion, although this does nothing to help in cases where the blade is jammed in bone (for which reason the British Army's first rule of bayonet fighting during the First World War - aim at the breastbone - was inappropriate).
In modern warfare, bayonets are rarely useful as weapons because most combat occurs at a distance. However a bayonet remains useful as a utility knife, and as an aid to combat morale. Despite the limitations of the bayonet, it is still issued in most armies and most armies still train with them. The modern sawback U.S. M9 Bayonet is issued with a special sheath designed to double as a wire cutter. The M9 Bayonet replaces both the M7 Bayonet of the 1960s and the Ka-bar fighting knive of WWII.
Modern bayonets are often knife-shaped with handles and a socket, or permanently attached to the rifle as with the SKS. The push-twist motion of fastening the modern bayonet has given name to several connectors and contacts including the BNC ("Bayonet Neill-Concelman") connector.