Bo Hi, also known as a fuller or a groove, refers to a longitudinal groove or channel that is sometimes present on the blade of a katana or other Japanese swords. It is a shallow, narrow groove that runs along the blade's surface, typically starting from the base of the blade (near the habaki) and extending toward the tip.

The primary purpose of Bo Hi is to reduce the weight of the blade while maintaining its structural integrity. By removing material along the groove, the overall weight of the katana is reduced, resulting in a lighter and more agile sword. This can improve the sword's handling and maneuverability, making it easier to wield and control during combat.

Additionally, Bo Hi can influence the harmonics and acoustics of the sword. When swung or unsheathed, the air passing through the groove creates a distinct sound, often described as a "sword whistle" or "sword hum." This adds to the aesthetic and auditory experience of wielding a katana.

Bo Hi also has a decorative aspect. The groove is often meticulously crafted and adds visual interest to the blade, enhancing its overall appearance and appeal.

It's worth noting that not all katanas have Bo Hi. Some blades are solid and do not feature a groove. The decision to include Bo Hi or not depends on the preferences of the swordsmith, the intended use of the katana, and the desired balance between weight reduction, handling characteristics, and aesthetic considerations.