The Science of Sori — Why the Curve of the Nihontō Arises and the Dimensional System of Each Part

What Is Sori?

Sori refers to the arc-shaped curve that forms in the blade of a nihontō. It is the single greatest morphological feature that distinguishes the nihontō from the straight sword (ken, chokutō), and was established from the mid-Heian period (around the 10th century) onward.

Sori is not merely an aesthetic element; it is directly tied to the practical functions of cutting power, drawing speed, and portability. Moreover, the formation of sori is an inevitable product of the physical process of yaki-ire (quenching).

The Physical Mechanism Behind the Formation of Sori

The sori of a nihontō arises primarily from thermal stress during yaki-ire (quenching).

Details of the Mechanism

1. Heating stage: The entire blade expands uniformly
2. Water-quenching stage:

• Edge section (thin clay): rapid cooling → martensitic transformation → volume expansion (approximately 1%)
• Body/mune section (thick clay): slow cooling → pearlitic transformation → minimal volume change

3. Onset of deformation: Because the edge section expands in volume through martensitic transformation, the blade curves toward the mune (back of the blade) side

This phenomenon is explained as "bending stress toward the mune side caused by martensitic volume expansion." The tōshō (swordsmith) bends the blade slightly in the reverse direction beforehand (uchisori) so as to predict and control the sori after yaki-ire.

Additional Factors Contributing to Sori

• Work hardening during tanren (forging): The stress states of the inner and outer layers differ through orikaeshi tanren (folding)
• Thinness of the edge section: Because the edge is thinner than the body, it has lower thermal mass and undergoes greater temperature change

Part Names and Dimensional System of the Blade

Each part of a nihontō has its own name and measurement standard.

Length Dimensions

Width and Thickness Dimensions

Sori Dimensions

Standard dimensions (approximate guide for a tachi of 2 shaku 3 sun 5 bu ≈ 71 cm class)

• motohaba: 29–32 mm
• sakihaba: 18–22 mm
• motokasane: 6–8 mm
• sori: 18–22 mm

Types of Sori and Their Functional Significance

Sori is classified into three types depending on which part of the blade the peak of the curve falls on.

Koshi-zori

Characteristic: The peak of the sori falls near the ha-moto (root of the blade) Period: Common in tachi of the late Heian to early Kamakura Function: Excels in downward cutting strokes on horseback. The center of gravity is closer to the hand, making it easy to control Representative: Kotō such as Ko-Bizen and Ko-Sanjō

Naka-zori

Characteristic: The peak of the sori falls near the center of the blade Period: Tachi and uchigatana of the mid-Kamakura to Nanbokuchō and Muromachi Function: Highly versatile, a shape that balances slashing and thrusting Representative: The standard shape across many periods

Saki-zori

Characteristic: The peak of the sori falls toward the kissaki (point) side Period: The "archetypal Enbun-Jōji style" and works such as Kikusui of the Nanbokuchō to Muromachi Function: Excels in thrusting and draw-cutting. The center of gravity is toward the tip Representative: Ōdachi and uchigatana of the Nanbokuchō

Historical Changes in Nagasa and Sori Over Time

The shape of the nihontō changed over time.

Conclusion

The sori of the nihontō is not a product of chance but an inevitable consequence of the physical phenomenon of yaki-ire (quenching), and at the same time a functionally optimized shape. Understanding the dimensional system of each part is indispensable foundational knowledge for appraising, purchasing, and collecting nihontō.