The hira-zukuri (pronounced "hee-rah zoo-koo-ree") is bascally a "V" sectioned blade with no ridges on the side. This design is the most popular of all historical tanto and aikuchi (knives) and is the design for most wakizashi (short swords).

Note that the above illustration for the sword is only generic in full sword length. Most hira-zukuri style blades are for shorter weapons. Because there are no side ridges as in the more common and popular shinogi-zukuri style, there is greater chance at bending at longer lengths since there is less material to resist lateral stress.

In terms of profile, the edge runs parallel and curves to form the tip. Blades will vary in how much curvature but the edge forming the tip follows the gentle arc of a circle, and the size of that arc will vary from one blade to another. Also wakizashi made of the hira-zukuri style aren't necessarily straight; the heat treatment using clay to achieve a harder edge and softer body will cause the blade to curve after the final water quench.

Historical tanto are mostly of the hira-zukuri style. These are a far cry from the crude "armor-piercing chisel point" descriptions as mispresented by knife industry marketing (Japanese knives did not need to pierce armor - eye slots could be reached, and one need only flip open the neck guard to access the soft neck flesh beneath in order to enact the kill.)

ILLUSTRATION BY ADRIAN KO