Kirihazukuri Blade Geometry: From Armor-Piercing Tanto to Wakizashi, the Practical Aesthetics of This Distinctive Cross-Section

What is Kiriha-Zukuri? A Japanese Sword Form

Kiriha-zukuri (kiriha-zukuri) is a type of sword construction (cross-sectional shape of the blade) that refers to a form created by significantly shaving away the spine side of shinogi-zukuri (shinogi-zukuri). In standard shinogi-zukuri, a flat surface called the munebira (munebira) exists from the mune (spine) to the shinogi-suji (ridgeline), but in kiriha-zukuri, this section is angled and cut away, creating a wide flat surface that extends directly from the spine toward the edge.

The name is believed to derive either from "construction emphasizing the kiriha (cutting edge)" or from "the construction of an edge with the spine side cut away." In any case, compared to standard shinogi-zukuri, the edge appears wider, and the cross-section has a characteristic appearance resembling a wedge (kusabi) shape.

Historical Background of Kiriha-Zukuri

Kiriha-zukuri swords are most commonly seen during the period from the Nanboku-chō period (1336–1392) through the early Muromachi period (early 15th century). This era marked a transition from the grand tachi of the Kamakura period to practical uchigatana, and was a time when various experimental blade forms were attempted.

During the tumultuous Nanboku-chō period, oversized swords such as ōtachi and notachi became popular, but at the same time, demand increased for tanto and wakizashi designed for greater responsiveness in close-quarters combat. Kiriha-zukuri appears to have developed as a form responding to close-combat demands, with particular appreciation for its shape, which could accommodate both thrusts into gaps in armor and horizontal slashing attacks.

Technical Characteristics of Kiriha-Zukuri

Kiriha-zukuri blades differ from standard shinogi-zukuri in the following ways.

Cross-sectional Shape: The cross-section of kiriha-zukuri shows a wide flat surface on the spine side, with the ridgeline corresponding to the shinogi-suji either absent or positioned very low. As a result, the gradient from spine to edge is gentle, and the entire width of the edge contributes to cutting ability.

Thickness of the Back: Kiriha-zukuri swords typically have a thicker kasane (spine thickness) than standard. The increased volume on the spine side raises blade rigidity and prevents edge chipping during cutting.

Edge Angle (Hakaku): The cross-sectional angle of the edge is greater than in standard shinogi-zukuri, giving it a cutting characteristic more of "pushing through" rather than "slicing." If the thin-edged shinogi-zukuri cuts with a clean "su" motion, kiriha-zukuri penetrates with a solid "dosu" bite.

Relationship to Naginata-Naoshi

The most commonly circulating kiriha-zukuri swords in modern times are naginata-naoshi (naginata-modified) wakizashi and tanto. Naginata-naoshi refers to swords created by shortening a naginata blade and converting it into a wakizashi or tanto, with the original naginata itself having a kiriha-zukuri cross-section.

Consequently, naginata-naoshi wakizashi and tanto retain pronounced kiriha-zukuri characteristics. Specifically, the following features are evident.

• The edge is proportionally wide relative to the blade width, with a thick spine
• The edge width changes abruptly from the hama-chi (edge-back junction) to the kissaki (point), with a wide base tapering to a narrow tip
• A hardening line called muneyaki (spine-hardening) may sometimes be visible on the spine as a remnant from the naginata era

Representative Uses of Kiriha-Zukuri

Kiriha-zukuri swords were well-suited to the following purposes.

Self-Defense Tanto: Emphasizing portability and practicality, kiriha-zukuri tanto were widely used from samurai to high-ranking merchants and monks. With a wide edge and ease of applying force, they were well-suited for emergency use.

Alternative to Armor Piercer: While not as slender as a pure yoroidōshi (armor-piercer), kiriha-zukuri wakizashi could be used both for thrusting at armor seams and for standard slashing, providing high versatility on the battlefield.

Ceremonial and Religious Offerings: Due to its visually distinctive presence, kiriha-zukuri was prized both as an offering to shrines and temples and as a ceremonial weapon for samurai. Examples of kiriha-zukuri wakizashi and tanto among surviving shrine-offered swords are not uncommon.

Extant Masterpieces of Kiriha-Zukuri and Points of Appreciation

The following are points to consider when appreciating kiriha-zukuri swords in museums or sword shops.

Confirming the Construction: When viewing the sword from the spine side, verify whether a wide flat surface extends continuously from the spine toward the edge. The low position (or absence) of the shinogi-suji is the defining characteristic of kiriha-zukuri.

Consistency with the Hamon: Kiriha-zukuri swords often feature suguha (straight) or kō-midare (small wavy) hamon. If an ornate chōji-midare (clove-leaf wavy) or gunome (scallop-shell) pattern is present, the original form may have been altered by later polishing.

Confirming Naginata Origins: In the case of naginata-naoshi, the nakago (tang) may bear inscriptions such as "naginata" or additional signatures indicating the original blade length or proportions. When this can be confirmed, the modification's provenance becomes clear, enhancing appraisal value.

Kiriha-zukuri swords, as a form embodying the diversity of Japanese sword construction, hold unique appeal as objects of collection and research. Being products of the historical modification known as naginata-naoshi, they serve as witnesses to a transformative era in sword history.