The yoroi-dōshi is an extra thick tantō, a long knife, which appeared in the Sengoku period (late Muromachi) of the 14th and 15th centuries. The yoroi-dōshi was made for piercing armour and for stabbing while grappling in close quarters. The weapon ranged in size from 20 cm to 22 cm, but some examples could be under 15 cm, with a "tapering mihaba, iori-mune, thick kasane at the bottom, and thin kasane at the top and occasionally moroha-zukuri construction". The motogasane (blade thickness) at the hamachi (the notch at the beginning of the cutting edge) can be up to a half-inch thick, which is characteristic of the yoroi-dōshi. The extra thickness at the spine of the blade distinguishes the yoroi-dōshi from a standard tantō blade.
Yoroi-dōshi were worn inside the belt on the back or on the right side with the hilt toward the front and the edge upward. Due to being worn on the right, the blade would have been drawn using the left hand, giving rise to the alternate name of metezashi (馬手差), or "horse-hand (i.e. rein-hand, i.e. left-hand) blade".
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