The hachiwari (鉢割 "splitting cap"), also known as kabutowari (兜割 "splitting helmets"), was a kind of stiletto, ("knife-dagger"), triangular section and blunt blade used in Japan in the feudal period.
Long about 30-35cm (although some longer versions reached 45cm), was curved and had a hook on the back, near the base of the handle.
In some cases the workmanship of the hachiwari was so similar to that of a tantō to be considered a real knife, rather than a blunt-bladed club.
Regarding its use, it is believed that it was very similar to that of the jitte: brought to the side as if it were a dagger, in combat was usually used together with the sword, held in the right hand, holding it in the left hand with the aim of parrying the blows of the opponent's weapon and try, if possible, to break the blade with the hook.
In alternative it could be used in order to break through the helmet or, as it appears more probable, in order to penetrate of tip in the interstices of the armor of the adversary.
In fact, it seems more legendary than anything else that samurai were able to break the kabuto of an O-yoroi with a hachiwari: the kabuto was made of several sheets of metal, mostly iron, riveted and nailed, which would require enormous pressure to be broken.
On the other hand, it would seem likely to be able to deform the helmet or to detach a piece by hooking it with the hook and levering.
Similar weapons were also known in Europe, where they were called left-handed daggers or sword-breakers.
However, even if the function and the way to use them were practically the same, their form was very different from that of the hachiwari: they were large double-edged knives in which one or even both blades were "comb" so that, parrying the opponent's blade, it was possible to block it between the teeth and, through a twist, try to disarm the opponent or even break the weapon.
It seems that Munisai Shinmen, father of Musashi Miyamoto, was a master in the use of the kabutowari to the point of receiving the compliments of shōgun Ashikaga for his ability.
Nowadays there are no known martial arts disciplines specific to the use of hachiwari, however some schools of Bujinkan provide for its teaching as an extension of jittejutsu.
Therefore, it is still possible to find hachiwari in some small Japanese gun stores.