A 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 20th centuries. When unmounted from a musket or rifle, sword bayonets with their typical hilts and long blades also could be wielded as short swords. While modern military bayonets typically have knife blades, they are usually too short to be called sword bayonets and are more akin to fighting/utility knives.
A typical example of an early sword bayonet is the 58 centimetre (23 inch) blade variety designed for the Infantry rifle, later called the Baker rifle of the Napoleonic era British Army. It was intended, when fitted to the bar on the side of the muzzle, to provide a more lethal defence against cavalry when combined infantry formed squares, and, it was hoped, a better hand-to-hand combat weapon when used detached. In truth, most riflemen found it worked better for cutting brush and roasting meat over a fire. (see Rifleman Harris, Costello's, Simmons's diaries) Riflemen also found that when the sword fitted, the Baker rifle became unbalanced and less effective for loading and then shooting accurately. Riflemen therefore removed the sword when shooting.
The German Schlachtmesser (butcher knife) was better in this respect. With the appearance of the hiltless sword bayonet, such as the socket-mounted variety, their use on the end of the musket or rifle also became a hindrance during the reloading of the muzzle-loaded longarm, (a common problem to all muzzle-loading infantry weapons). A bayonet of similar style and dimension was used on the Lee-Enfield rifle of the early 20th century.
However, the advantages of sword bayonets over spike bayonets are evident. Where a spike bayonet turns the rifle into a spear, a sword bayonet turns it into a glaive. Unlike spike bayonets, which can be used only for thrusting, sword bayonets can also be used for slashing. Twisting a sword bayonet in the wound was especially lethal. Before the advent of modern medicine after WWI, a soldier struck by a sword bayonet was very unlikely to survive.
While most sword bayonets have straight blades, a popular variant in the 19th century featured sinuous, S-curved blades like those found on the Balkan's and Middle-East's sword called the yataghan. Today, sword bayonets of this style are said to have "yataghan" blades, or to be "yataghan-bladed".
Both sword and spike bayonets lost their popularity after World War I. While sword bayonets can be effective as short swords, they proved to be too unwieldy in cramped quarters in trench warfare, although spike bayonets continued to be used throughout most of the 20th century. A shorter version of the sword bayonet, the knife bayonet, was developed. Today, the majority of modern bayonets are knife bayonets.