What Is a Bo-hi? 棒樋 — The Katana Groove (Fuller) Explained
The bo-hi (棒樋) is a single wide, straight groove — a fuller — carved lengthwise along the upper part of a Japanese blade, running through the shinogi-ji from below the tip toward the tang. Its name means "stick groove," describing its long, even, bar-like form. A bo-hi is the most common of the many hi (groove) styles, and it serves both a practical structural purpose and an aesthetic one, making it a familiar sight on katana, tachi and naginata.
For a collector, a bo-hi is worth understanding because it changes how a blade behaves and how it is valued. It lightens and balances the sword, produces the characteristic sword-cut whistle, and — because it is a carving cut into the steel — its execution and later alteration carry real information about a blade's history and condition.
What the bo-hi is for
A groove is not merely decorative. The bo-hi serves several concrete functions:
- Weight reduction — removing a channel of steel from the upper blade lightens the sword and shifts the balance point back toward the hand, improving handling without seriously weakening the blade (the groove sits in the softer shinogi-ji, not the edge).
- Rigidity — like an I-beam, the grooved cross-section resists bending relative to its reduced weight.
- Tachikaze (太刀風) — the "sword wind," the audible whistle a grooved blade makes when it cuts cleanly through the air. Practitioners use it to judge whether a cut has correct edge alignment.
The groove is cut and finished by hand, and on antique blades it was often carved by the smith or a specialist carver, then refined by the polisher.
Bo-hi variations and related grooves
The bo-hi is one member of a family of hi. A collector should recognise the common variations:
- Bo-hi (棒樋) — the single wide straight groove, the baseline form.
- Bo-hi ni tsure-hi (棒樋に連れ樋) — a bo-hi accompanied by a slim companion groove running alongside it.
- Soe-hi (添樋) — the narrow companion groove itself, cut parallel to the main bo-hi.
- Futasuji-hi (二筋樋) — two parallel grooves of similar width.
The way a groove ends is also read: it may run out to a rounded or square stop, and how it terminates in the tang area and near the tip is a period and school clue. A hi that ends cleanly and travels the full length as expected is a sign of an unaltered blade.
Hi versus horimono, and what a collector reads
It is important to distinguish grooves from carvings. A hi is a functional groove; a horimono (彫物) is a decorative or religious carving — a dragon, a deity, Sanskrit bonji, or a ken — engraved into the blade. Both are cut into the steel, but they serve different ends, and a blade may carry either, both, or neither.
For buyers, the key condition question is whether a groove is original or added later. A bo-hi carved after forging can be used to hide a forging flaw or edge damage, and a groove added to a shortened (suriage) blade may run awkwardly into the tang. A groove that has been deepened or re-cut by an amateur can thin the blade and hurt value. On a healthy sword the bo-hi is even in width and depth, cleanly polished inside, and consistent with the blade's age and school — details worth confirming before purchase.
Frequently asked questions
What is the purpose of a bo-hi groove on a katana?
A bo-hi (棒樋) lightens the blade and shifts its balance toward the hand, stiffens the cross-section like an I-beam, and produces the tachikaze whistle that signals a clean, correctly aligned cut. It sits in the softer shinogi-ji, so it lightens the sword without weakening the edge.
Is a bo-hi the same as a blood groove?
No. "Blood groove" is a myth; the bo-hi is a fuller cut for weight reduction, balance and rigidity, not for blood flow. Its real functions are structural and, through the tachikaze whistle, diagnostic of good cutting technique.
What is the difference between a hi and a horimono?
A hi (樋) such as the bo-hi is a functional groove that lightens and stiffens the blade. A horimono (彫物) is a decorative or religious carving, such as a dragon, deity or ken. Both are cut into the steel, but one is structural and the other artistic.
Does a bo-hi affect a sword's value?
It can. An original, cleanly executed bo-hi is a normal and desirable feature. But a groove added later can conceal a flaw, and one deepened or re-cut by an amateur thins the blade and lowers value, so its originality and execution should always be checked.