The rope or cord found on a katana is called a "sageo." The sageo serves multiple purposes and has both functional and decorative uses. Here are its main functions:

1. Securing the Saya: The primary purpose of the sageo is to secure the katana's saya (scabbard). It is tied around the kurikata (knob) on the saya, helping to keep the scabbard in place and preventing it from accidentally sliding out or being dislodged during movement or transportation.
2. Carrying and Handling: The sageo can be used to create a loop or strap, allowing the katana to be worn around the waist or shoulder. By attaching the sageo to the saya, the sword can be carried securely, keeping it close at hand. The sageo provides added security and convenience during transportation or when the sword needs to be quickly accessed.
3. Aesthetic Enhancement: The sageo also serves a decorative purpose. It adds visual interest and can be chosen to complement the color scheme or style of the katana. The sageo can be made from various materials such as silk, cotton, or synthetic fibers and may feature different patterns or weaves. It is an opportunity for personalization and customization of the sword's appearance.

It's important to note that the tying of the sageo is considered a part of the traditional Japanese sword culture and can vary in knot style and method. Different knot styles have their own names and symbolism. The proper tying of the sageo is often learned and practiced by enthusiasts of Japanese martial arts or swordsmanship.

While the sageo primarily serves functional and decorative purposes, it does not affect the sword's performance or functionality in combat. Its main role is to secure the saya and facilitate the carrying and handling of the katana.