Kasane, Motohaba, and Sakihaba Design: How Cross-Sectional Taper Affects Sharpness and Strength in the Japanese Sword

Kasane, Motohaba, and Sakihaba — The Three Dimensions of Blade Cross-Section

When describing the blade shape of a Japanese sword in numerical terms, three dimensional elements are indispensable.

• Kasane (blade thickness): The thickness from the mune (back of the blade) toward the cutting edge. Measured separately as moto-kasane near the base and saki-kasane near the kissaki (point).
• Motohaba: The width of the blade base in the direction of the cutting edge. Refers to the dimension at the habaki-moto.
• Sakihaba: The width near the kissaki. Formally measured at the yokote-shita position.

The combination of these three elements defines the blade's "cross-sectional taper," and influences every aspect of sharpness, center of gravity, strength, and handling. They are the starting point for reading a sword through its measurements.

Typical Values for Each Dimension — Changes by Period

The trends in kasane and width have shifted significantly over the centuries. Those changes are a mirror reflecting transformations in combat style, materials technology, and aesthetic sensibility.

Heian-Kamakura tachi (kotō period)

The tachi of this period, designed primarily for slashing strikes by mounted warriors, has a slender, tapered "supple" form. With a narrow sakihaba, the kissaki side is lighter, making balance easier to achieve.

Nanbokuchō-period ōdachi and nagamaki

The "robust form" exemplified by Nanbokuchō-period ōdachi and nodachi was the result of prioritizing destructive power in mass infantry combat. Motohaba, sakihaba, and kasane are all large, and the weight is estimated to have been considerable even by modern standards.

Muromachi–Sengoku uchigatana (practical sword period)

These are dimensions optimized for the uchigatana suited to infantry combat. By maintaining a moderate difference between motohaba and sakihaba while placing the center of gravity near the mono-uchi, striking power in actual combat is secured.

Edo-period koshirae swords (art and formal dress sword period)

This was a period when the practical demand of warfare had faded, and appreciation and formal dress became the focus. Blade shapes shifted toward a slender, thin "elegant form."

Taper Ratio — What the Sakihaba/Motohaba Ratio Reveals

More than the dimensions themselves, what most directly reveals the character of a sword is the ratio of sakihaba to motohaba (taper ratio).

Taper ratio = sakihaba ÷ motohaba

The smaller the taper ratio (steeper taper), the more the following characteristics emerge.

• The kissaki becomes lighter, enabling faster handling
• Higher thrusting ability at the tip
• The overall balance of the sword is good and it is easy to handle

Conversely, the larger the taper ratio (more gradual taper), the more the character shifts as follows.

• The overall weight of the sword increases, enhancing striking power
• Slashing near the tip is powerful
• Handling becomes heavier

Kasane Taper — The Difference Between Moto-kasane and Saki-kasane

Equally important alongside width is the taper of kasane (thickness). The rate of change from moto-kasane to saki-kasane determines the stiffness distribution of the blade.

Typical moto-kasane/saki-kasane ratios:

• Typical uchigatana: moto-kasane 7 mm → saki-kasane 4 mm (ratio approximately 0.57)
• Robust Nanbokuchō sword: moto-kasane 10 mm → saki-kasane 6 mm (ratio approximately 0.60)

The steeper the kasane taper, the more rapidly the blade narrows toward the tip, reducing air resistance during thrusting and lunging. Conversely, a gradual taper preserves overall blade stiffness, making it more resistant to clashing. Combined with the width taper, the character of the sword is tuned with even finer precision.

Realization Through Forging — The Difficulty of Dimensional Control

Accurately reproducing a designed kasane and width taper through hammer blows is no easy task. Let us trace the key points the tōkō must keep in mind at each stage.

Establishing the Basic Form Through Sonobe

The steel completed through orikaeshi tanren (folding) is drawn out in the "sonobe" stage to establish the blade's fundamental length, width, and kasane. From this stage, hammering with the motohaba and sakihaba taper already in view is required.

Precision Shaping Through Hizukuri

In the "hizukuri" stage following sonobe, the shape of each part of the blade is refined with precision. Three operations proceed simultaneously here.

• Hammering from the mune direction to reduce kasane
• Hammering from the edge direction to adjust width
• Varying the force and angle of hammering for each section to create the taper

Deformation Correction After Yaki-ire

During yaki-ire (water quenching), thermal stress can cause slight deformation of the blade. When correcting this deformation through the "hizumitori" (straightening) technique explained in sori control techniques, it is necessary to proceed carefully while taking into account the effects on kasane and width.

Conclusion — Invisible Design Determines the Character of the Sword

The taper design of kasane, motohaba, and sakihaba does not directly show up in the finished sword's appearance. Yet it is an essential design parameter that governs every aspect of sharpness, center of gravity, handling, and strength.

The fact that optimal dimensions have shifted from period to period is itself proof that the Japanese sword has continually been updated in response to the demands of actual combat, advances in materials, and changes in aesthetic sensibility. The tōkō of today also weave the blade's taper into each hammer blow, drawing on a sense of proportion learned from centuries of accumulated knowledge. For those wishing to compare these cross-sectional differences in actual works, the online catalog allows observation of blade shapes from multiple periods side by side.