What is a Bo-Hi (Blood Groove)?

The katana is one of the most famous swords in the world. It originated in feudal Japan and is characterized by a distinctive single-edged curved blade with a long handle to support wielding with two hands. Japanese swordsmiths invested a lot of energy and time in perfecting the katana, adding intricate features that often go unnoticed. One of these features is blood grooves, which we will discuss today.

A “Bō-Hi” (blood groove) is a long, straight, and wide groove on a Japanese blade.

Bo-Hi – The Blood Groove of a Blade

A blood groove, also known as a blood rill or hollow grind, is a narrow indentation that runs the length of the katana through the center of the blade. The blood groove is a depression in the steel. However, the terms “blood rill” and “blood groove” can be somewhat misleading; when a Japanese katana has such a notch, it has nothing to do with blood. Instead, it is used to make the blade lighter and more flexible.

Advantages of a Blood Groove

So, why were blood grooves a common feature in traditional Japanese katana?

• Reduced Weight
• Increased Agility in Sword Handling
• Aesthetics

Aside from the aesthetic benefits that a bo-hi offers, there’s a very real, practical reason for its usage. A blood rill adds extra agility, owing to the overall reduced weight of the blade. Granted, a small indentation in the middle of a katana might not seem like a significant weight reduction, but it can add up, especially considering that blood rills run the entire length of the blade.

According to Wikipedia, blood grooves – in combination with heat treatment and blade hardening processes – can reduce the weight of a katana by up to 35%. Normally, methods to reduce the weight of a sword diminish its power, but with a blood groove, the sword retains its original strength and stability. However, it is important to note that the benefits of blood grooves decrease as the blade is shortened (e.g., longer katana benefit more from blood grooves than shorter katana).

Customize Your Own Katana

In our KATANZO Forge, you can have Your Own Katana Crafted, and of course, you also have the choice whether the blade of your samurai sword has a blood groove (Bo-Hi) or not.