The little decoration on a katana is known as a tsuba. The tsuba is the handguard or guard of the katana, which is situated between the blade and the hilt. It serves as a protective component and also contributes to the overall aesthetic design of the sword.

The tsuba is typically a round or oval-shaped metal plate with a central opening through which the blade passes. It is often intricately designed and can feature various motifs, symbols, or patterns that reflect the artistic style and cultural influences of the time.

The primary function of the tsuba is to protect the wielder's hand from sliding onto the blade during combat, preventing injuries and improving grip. It also helps to balance the sword and adds weight near the guard, enhancing the overall handling and maneuverability.

Apart from its functional purpose, the tsuba is considered an important element of the katana's artistic expression. Tsuba designs can vary widely, incorporating motifs from nature, mythology, historical events, or geometric patterns. The craftsmanship and aesthetics of the tsuba are often appreciated and admired by collectors and enthusiasts of Japanese swords.

In addition to the tsuba, other components of the katana's hilt, such as the menuki (ornamental grip decorations) and fuchi-kashira (collar and pommel cap), also contribute to the overall decoration and design of the sword.