Katana Bo-hi Guide — The Fuller That Lightens and Strengthens the Blade
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Introduction
The bo-hi (棒檋, fuller or groove) is one of the most recognizable features of many katana — a long groove running along the upper portion of the blade that reduces weight while maintaining structural strength. The bo-hi is both functional and aesthetic — it lightens the blade, produces a distinctive sound when swung, and adds visual interest to the blade's surface.
Browse our handmade katana collection for blades with and without bo-hi.
What Is a Bo-hi?
The bo-hi is a groove carved into the surface of the blade — typically running from near the habaki (blade collar) to near the kissaki (tip). The groove removes material from the blade, reducing weight by 100–200g without significantly reducing structural strength.
The bo-hi is carved after the blade is forged and before it is polished — a specialized skill that requires precise control to maintain consistent depth and width along the full length of the groove.
Types of Bo-hi
Bo-hi (棒檋) — Single Fuller: A single groove running along the upper portion of the blade. The most common type — found on the majority of katana with fullers.
Futasuji-hi (二筋檋) — Double Fuller: Two parallel grooves running along the blade. Less common than the single bo-hi — associated with some historical schools and styles.
Kuichigai-hi (くいちがい檋) — Staggered Fuller: Two grooves of different lengths, staggered along the blade. A distinctive and relatively rare configuration.
Naginata-hi (肖檋) — Naginata Fuller: A wider, shallower groove associated with naginata blades. Sometimes found on katana in the naginata style.
The Sound of the Bo-hi
One of the most distinctive features of a bo-hi katana is the sound it produces when swung — a distinctive "fuu" or whistling sound caused by air moving through the groove. This sound is called the "wind sound" (kaze no oto) and is considered aesthetically pleasing in Japanese sword culture.
The sound of the bo-hi also serves a practical purpose — it provides auditory feedback on the quality of the cut. A clean, powerful cut produces a clear, resonant sound; a poor cut produces a dull or irregular sound.
Bo-hi vs No Bo-hi — Which Is Better?
The choice between a bo-hi and no bo-hi katana depends on your priorities:
With bo-hi: Lighter (100–200g less), faster, produces distinctive sound, more visually complex. Preferred for iaido and fast-draw techniques where speed is paramount.
Without bo-hi: Heavier, more cutting power, simpler visual appearance. Preferred for tameshigiri where cutting power is paramount.
Neither is objectively better — the choice depends on your intended use and aesthetic preferences.
FAQ
Q: What is a bo-hi?
A bo-hi is a groove carved into the blade surface that reduces weight by 100-200g while maintaining structural strength.
Q: Does a bo-hi weaken the blade?
No — a properly executed bo-hi does not significantly weaken the blade. The groove removes material from the thickest part of the blade where structural strength is least critical.
Q: What sound does a bo-hi make?
A distinctive whistling or "fuu" sound when the blade is swung — caused by air moving through the groove.
Q: Should I choose a katana with or without a bo-hi?
With bo-hi for iaido and speed; without bo-hi for tameshigiri and cutting power. Both are valid choices depending on your priorities.
Q: What is a futasuji-hi?
A double fuller — two parallel grooves running along the blade. Less common than the single bo-hi.
Summary
The bo-hi is a functional and aesthetic feature that lightens the blade, produces a distinctive sound, and adds visual interest. Understanding bo-hi types, the sound they produce, and the trade-offs between bo-hi and no bo-hi helps collectors choose the right katana for their needs.