The bo-hi (pronounced boh-hee) is the long, smooth channel carved down the flat of a katana blade, just below the spine. In English it is usually called the fuller. The name joins bo, a staff or rod, with hi, a groove, and that is a fair picture: a single straight runnel that follows the blade from near the collar toward the point.
What it is not
The most stubborn myth is that the bo-hi is a “blood groove.” It is nothing of the sort. The fuller is an engineering feature, cut or ground into the steel after forging. By removing metal from the centre of the blade, it lightens the sword and shifts the balance back toward the hand, while the remaining ridges, the shinogi line above and the body below, keep the blade stiff. Think of it the way an I-beam carries load with less steel than a solid bar. Less mass, similar spine, a faster sword.
How to recognise quality
A good bo-hi is even in depth and width along its whole length, with crisp walls and a clean, polished interior, and it terminates in a graceful rounded or pointed tail rather than a ragged stop. Run a fingertip along it: it should feel uniform, not wavy. There is also the famous test the groove makes possible. A properly cut bo-hi catches the air during a swing and produces the clean whistle, the tachikaze, that tells a practitioner their edge alignment is true. A blade that whooshes raggedly or not at all is telling you something about either the cut or the technique.
Why it matters to you
If you train with repetitive cutting or solo forms, a bo-hi makes a katana quicker in the hand and gives you that audible feedback, which is why it suits a lively battle-ready katana. If you want maximum rigidity for heavy target cutting, a blade without a fuller carries more mass behind the edge. Browse the full katana collection to compare, learn the metal behind it in our steel guide, or keep exploring the anatomy of the blade.