A knife bayonet is a knife which can be used both as a bayonet, combat knife, or utility knifeSeitengewehr 1884/98, would go on in use until 1945 in German service.
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 ).
M9 Knife Bayonet
Modern knife bayonets have 6 inch to 8 inch long and .75 inch to 1.5 inch wide blades, and are often (thanks to @Jpegger for texture and help)