Kakuuchi Tanto: The Rare Square Cross-Section Blade Form and Its Surviving Cultural Properties

What is Kakuuchi?

The construction (tsukurikomi) of a Japanese sword refers to the cross-sectional shape of the blade. A typical Japanese sword takes the form known as shinogi-zukuri, with the cross-section forming a convex lens shape centered around the shinogi ridge. In contrast, kakuuchi is a special construction where the blade's cross-section takes a square or rectangular shape.

Kakuuchi is particularly rare among Japanese sword construction types and is rarely seen in standard uchigatana or tachi. It is primarily applied to tanto, and is called kakuuchi-tanto or kakugata-tanto.

Morphological Characteristics

The greatest characteristic of kakuuchi-tanto is that the cross-section maintains a square shape from the hamachi to the kissaki. While hira-zukuri tanto consists only of flat surfaces front and back, kakuuchi has the side surfaces of the blade (the faces connecting the mune and ha) finished flat as well, presenting an overall rectangular prism-like appearance.

The mune is finished with even sharper angles than the standard kakumune. The kasane (thickness) is relatively thick, and it is also interpreted as being designed with the practical purpose of piercing through gaps in armor, as armor-piercing tanto.

Historical Background and Production Period

Kakuuchi-tanto are said to have been produced primarily during the late Kamakura to Nanboku-chō periods. This was an era when large blades such as ōdachi and nodachi flourished, while short blades for close combat also diversified. Among the development of purpose-specific tanto such as yoroizushi, koshigatana, and kaiken, it is believed the kakuuchi form was adopted to maximize thrusting performance from the front.

It has high affinity with the thrust-specialized design philosophy favored by swordsmiths of the Sōshū tradition, and kakuuchi-tanto attributed to Sōshū-den exist. Additionally, some kakuuchi-tanto by swordsmiths of the Yamato tradition have been confirmed, and examples not limited to specific regional schools remain.

Extant Cultural Properties

The number of extant kakuuchi-tanto is very small, and some have been designated as Important Cultural Properties or Special Important Swords. They are housed in multiple domestic museums and art museums, and are occasionally displayed in Touken Ranbu content exhibitions and special exhibitions.

In evaluations by the NBTHK, kakuuchi is frequently noted as a rare form, and is explicitly marked as "kakuuchi" in the tsukurikomi section of appraisal certificates. Intact examples (without damage and with minimal wear from polishing) receive especially high evaluation.

Appraisal Points

When appraising kakuuchi-tanto, the following points are examined:

• First, check the cross-sectional shape—whether the cross-section viewed from the mune direction clearly forms a square.
• Second, examine the kasane thickness, which is characteristically thicker and heavier compared to hira-zukuri or shinogi-zukuri.
• Third, examine the relationship between the hamon position and kakuuchi—how the clay placement of the hardening is applied relative to the square cross-section demonstrates the craftsman's skill.

When there is suspicion of later alteration, checking for unnatural tooling marks on the mune shape and finish near the shinogi also becomes a confirmation point.

Revival by Contemporary Swordsmiths

In recent years, kakuuchi-tanto have also become subjects of revival and research by contemporary swordsmiths. Because it is a rare form, there are few opportunities to make them, and some swordsmiths are engaged in rediscovering the technique and reproducing examples. Some swordsmiths choose kakuuchi as their work for exhibition, and this ancient rare form brings new context to contemporary sword art.