In the context of a katana, the sageo refers to a woven cord that is tied around the saya (scabbard) of the sword. The sageo serves both functional and decorative purposes. Here are its main uses:

1. Securing the Saya: The sageo helps to secure the katana's saya, keeping it firmly in place and preventing accidental slippage or detachment. It is typically tied in a specific knot and looped around the kurikata (knob) on the saya.
2. Carrying and Handling: The sageo facilitates the carrying and handling of the katana. It can be used to create a loop or strap, allowing the sword to be worn around the waist or shoulder. The sageo helps keep the katana close at hand and provides added security during movement.
3. Aesthetic Enhancement: The sageo contributes to the overall aesthetics of the katana. It can be made from various materials such as silk, cotton, or synthetic fibers and is often designed to complement the color scheme or style of the sword. The sageo adds visual interest and can be an opportunity for personalization or customization.

Tying the sageo properly is considered a part of the traditional Japanese sword culture and is often learned and practiced by enthusiasts of Japanese martial arts or swordsmanship. There are different knot styles and tying methods for the sageo, each with its own significance and symbolism.

It's worth noting that the sageo is not exclusive to katanas and can be found in other traditional Japanese swords, such as the wakizashi and tanto. The sageo serves a similar purpose across these sword types, aiding in the secure carrying and handling of the weapon while adding an aesthetic element.