The bayonet is large knife or dagger that can be attached to the muzzle of a rifle to convert it into a weapon for stabbing and slashing.
The name probably derives from the French city of Bayonne, home of Marechal de Puysegur, who is said to have invented the bayonet about 1640. There is evidence, however, that it had previously been adapted to the harquebus and other early firearms. With the invention of such weapons it was a logical step to affix a metal lance to the firearm. With little alteration, plugged into musket muzzles in the 17th century.
The bayonet used by Puysegur's troops when they took Ypres for King Louis XIV in 1647 was a steel dagger with a cross guard and wooden haft or grip that was tapered to fit into musket muzzles of various calibers. This plug-type bayonet was often too loose or tight, and prevented firing of the musket.
A bayonet attached to the muzzle by two loose-fitting rings appeared in the late 1600's, and this was supplanted by the closer-fitting socket-bayonet, adopted between 1697 and 1703 by the English, German, and French. Not until 1800, however, were the socket locks secured by means of locking springs.
In the course of their development, bayonets have varied greatly in length and shape. The regulation bayonet of United States troops in the late 1960s was 8 inches (20.3 cm) long. The modern U.S. Army bayonet is 14 7/16 inches (37 cm) long, with a blade length of 10 inches (25 cm). It has a handgrip so that it can be used as an all-purpose knife, and when it is not in use it is carried in a sheath.
The Rusty Bayonet Myth
I had always heard that a rusty bayonet was a violation of the Geneva Convention, apparently not. After searching for a mention of it all I could find in relation to bayonets was in the Protocol Additional to the Geneva Conventions of 12 August 1949, and relating to the Protection of Victims of International Armed Conflicts:
"Part III : Methods and means of warfare" that you're not allowed to use "bayonets with a serrated edge".
So there you have it, you can use a bayonet with rusty blade. Just make sure it's not serrated.