The Bo-Hi - Parts of a Japanese Katana
The hi or bo'hi is the groove in the blade located just under the spine. This groove has been misunderstood by many as does the fuller in the European sword. Many of you have probably heard the term “blood groove” and how the groove existed to allow blood to pass from the victims body reducing suction so the blade does not get stuck. Some of you may have even told this story, but as fantastic as that sounds it is completely false. The “hi” like the fuller's purpose is to lighten the blade while compromising it's strength as little as possible. The more weight that is reduced will in turn create a faster blade and move the balance point closer to the wielder. The groove can be made at many different lengths and will sometimes extend straight through the habaki. Most people using the katana for heavy cutting opt to leave out the “hi” to not reduce any strength and bring the balance point forward resulting in a slightly harder hitting cut. The bo'hi is commonly preferred by modern martial artists that do many demonstrations or kata. The groove creates the classic audible “swish” sound during a cutting motion like we are used to hearing in the movies. It can make the demonstration sound more impressive and entertain an audience.
Example of a Bo-Hi