The hamon is a distinctive visual pattern that appears on the blade of a Japanese katana. It refers to the line or temper line that separates the hardened cutting edge (yakiba) from the softer, more flexible spine (mune) of the blade. The hamon is a result of the differential hardening process used in traditional Japanese sword smithing.

The creation of the hamon involves heating the blade to a high temperature and then rapidly cooling it, typically by quenching it in water or oil. The specific temperature control and the manner in which the blade is cooled contribute to the formation of the desired hamon pattern.

The hamon can take various forms and patterns, each with its own aesthetic appeal. Common hamon patterns include:

1. Straight Hamon (Chu-midare): This pattern consists of gentle undulations or irregularities along the blade.
2. Notare Hamon: This pattern features large, graceful waves that flow along the blade.
3. Gunome Hamon: This pattern comprises repeated semi-circular or irregular shapes resembling links of a chain.
4. Suguha Hamon: This pattern is characterized by a straight, uninterrupted line.

The hamon not only adds visual appeal to the blade but also serves as an indicator of the quality and skill of the swordsmith. The precise formation of the hamon requires a deep understanding of heat treatment and blade geometry. The hamon is often considered one of the most beautiful and distinguishing features of a well-crafted katana.