Kasane and Blade Width Taper Design: How Cross-Section Taper Affects the Cutting Performance and Strength of Japanese Swords

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

When describing the physical form of a Japanese sword numerically, three measurement elements are most important:

• Kasane (重ね): The measurement of the thickest part of the blade (the thickness from the ridge (mune) toward the edge direction). Measured separately as motokasane (元重ね) and sakikasane (先重ね).
• Motohaba (元幅): The width in the direction of the edge at the base of the blade (the moto). Refers to the width of the part called the habakimoto (はばき元).
• Sakihaba (先幅): The width near the blade tip (kissaki). Formally refers to the width below the yokote (横手下).

These three elements determine the "cross-sectional taper (narrowing shape)" of the blade, affecting all aspects of cutting ability, center of gravity, strength, and operability.

Typical Measurements by Period—Changes Across Eras

The kasane and width of Japanese swords changed dramatically across different periods. These changes reflect the combat styles, material technologies, and aesthetic preferences of the time.

Heian to Early Kamakura Tachi (Koto Period)

The tachi of this era were designed primarily for slashing from horseback, with a slender, tapering form that is flexible. The narrow sakihaba makes it easy to balance at the tip.

Large Tachi and Nagamaki of the Nanboku-chō Period

The "grand form" of the Nanboku-chō period resulted from prioritizing destructive power in massed infantry combat. Both motohaba and sakihaba are large, with thick kasane. The weight is estimated to have been considerable even by modern standards.

Uchigatana of the Muromachi to Sengoku Periods (Practical Blade Period)

Measurements optimized for the "uchigatana" size suitable for foot combat. Maintaining a moderate difference between motohaba and sakihaba, with the center of gravity positioned forward on the blade (near the monouchi), ensures cutting power in actual combat.

Edo Period Presentation Swords (Art and Formal Blade Period)

In an era that valued appreciation and formal use over practical combat, blade form shifted to a slender, thin "refined appearance."

Taper Ratio—What the Sakihaba/Motohaba Ratio Reveals

The ratio of sakihaba to motohaba (taper ratio) is an important numerical indicator of a blade's character:

Taper Ratio = Sakihaba ÷ Motohaba

The smaller the taper ratio (steeper taper):

• The tip becomes lighter, allowing faster manipulation
• Thrusting ability at the tip is higher
• The entire blade is easier to handle with good balance

The larger the taper ratio (gentler taper):

• The weight of the entire blade increases, raising cutting power
• Slashing near the tip is more powerful
• Manipulation becomes heavier

Kasane Taper—The Difference Between Motokasane and Sakikasane

Beyond width, taper in kasane (thickness) is also important. The rate of change from motokasane to sakikasane determines the stiffness distribution throughout the blade.

Typical ratio of motokasane/sakikasane:

• Standard uchigatana: motokasane 7mm → sakikasane 4mm (ratio approximately 0.57)
• Grand Nanboku-chō blade: motokasane 10mm → sakikasane 6mm (ratio approximately 0.60)

A steep kasane taper causes the blade to narrow rapidly toward the tip, reducing air resistance during thrusting. Conversely, a gentler taper preserves overall blade stiffness, making it more resistant to impacts.

Realization in Forging—The Difficulty of Dimension Control

Accurately realizing the designed kasane and width taper through forging (hammer work) is not easy.

Sonobe—Establishing Basic Blade Shape

After completing repeated folding, the steel material is drawn out in a process called "sonobe" to determine the basic length, width, and kasane of the blade. At this stage, tapping is performed with awareness of the taper between motohaba and sakihaba.

Hizukuri—Precision Shaping

After sonobe, "hizukuri" refines the shape of each part of the blade. At this stage:

• Tapping from the ridge direction reduces kasane
• Tapping from the edge direction adjusts width
• By varying the force and angle of tapping for each section, taper is created

Deformation Correction After Hardening

During hardening (water quenching), thermal stress from heat treatment can cause slight blade deformation. This deformation is corrected in a process called "hizumi-tori" while considering effects on kasane and width.

Conclusion—Invisible Design Determines Blade Character

The taper design of kasane, motohaba, and sakihaba does not directly appear in the finished blade's exterior, but is a fundamental design parameter that influences all aspects of the blade's cutting ability, center of gravity, operability, and strength.

The fact that optimal dimensions changed across different eras demonstrates that Japanese swords continually evolved in response to practical combat demands, material advancement, and changing aesthetic preferences. Modern swordsmiths, drawing on the dimensional sensibilities accumulated over centuries, embed this knowledge in each hammer strike to create the blade's taper.