Like other bladed weapons developed across the world, the Japanese Katana was designed to be flexible, strong and resilient. For example, the swordsmiths’ use of “shingane,” the soft and flexible form of tamahagane, gives the katana its resilient flexibility. In contrast, the use of “kawagane,” the harder tamahagane that makes up the outer casing of the sword metal, provides the sharp cutting edge that has become legendary.
Although we are mesmerized by the construction of the katana and may be tempted to assign otherworldly qualities to it, the Japanese Sword is nothing more than the absolute perfection of craft through trial and error, eventually resulting in unmatched metallurgical skill. There are, however, many components of the Japanese Sword that are shared with bladed weapons found around the world.
Today we’ll focus on the "hi" (pronounced "hee") or groove that is sometimes found on Japanese Swords. One might hear this referred to as a “blood groove,” but this interpretation is misguided; the history of the hi is found in the technology of the blade itself. Initially, these grooves were carved into blades to increase blade rigidity. If we looked at a cross section of a blade with hi, we would see that the shape resembles a modern day “I” beam, a shape developed due to its incredible strength-to-weight ratio. Hi performed much of the same function on a blade as “I" beams perform in modern building materials: the shape increases rigidity while reducing weight. We can see that the "hi" design's functionality was also championed in bladed weapons from around the world. The fuller groove (pictured below), similar to the Japanese hi, is present in many European weapons for the exact same purpose - blade rigidity and weight reduction.