What part are you asking about, the kurigata, the sageo, saya? The kirigata is a knob with a hole in it to keep the sageo on the saya, the sageo is to help keep it in your obi and I have also heard it was used to tie back your sleeves, the saya is the scabbard and protects you and the katana.
The following are the detailed introduction and functions of these three accessories
The Kurigata (くりがた)
Function: The kurigata is a small, knob-like projection located on the side of the saya (鞘) near the opening. Its primary role is to secure the sageo (下绪) in place. By wrapping the sageo around the kurigata, the sword can be firmly attached to the wearer’s obi (帯) — a traditional belt — preventing it from slipping or moving during movement. Additionally, the kurigata often serves as a decorative element, with intricate carvings that reflect craftsmanship or the owner’s personal style, adding both utility and artistry to the weapon.
The Sageo (さげお)
Function: The sageo is a long, braided cord attached to the kurigata (栗形) of the saya (鞘). Its key function is to secure the katana to the wearer. When wearing the sword, the sageo is wrapped around the obi (帯) multiple times and tied tightly, ensuring the katana remains stable even during intense actions like combat or rapid movement. Beyond practicality, the sageo holds cultural significance: its length, color, and braiding pattern can indicate the wearer’s social status, clan affiliation, or personal taste. In traditional settings, it may also be used to hang the sword when not in use, keeping it safe and accessible.
The Saya (さや)
Function: The saya is the scabbard of the katana, typically crafted from wood (most commonly magnolia) and coated with layers of lacquer for durability and water resistance. Its primary role is to protect the blade (刀身，katana no ha) from damage, rust, and contamination. When the sword is not in use, the saya shields the sharp edge from scratches, moisture, and debris, preserving its integrity over time. Additionally, the saya contributes to the katana’s balance and portability, allowing for safe and comfortable carrying. A well-fitted saya ensures a smooth draw (抜刀，nukitsuke) and secure retention when sheathed, making it an essential component of both function and tradition.