A kaiken (懐剣) is a single- or double-edged tantō with a length of 15-20 cm, devoid of ornamental features and housed in a simple mounting.
Uses of Kaiken
In the past, it was carried by both men and women of the samurai class in Japan. It was useful for self-defense in indoor spaces where the long uchigatana and the intermediate-length wakizashi were inconvenient.
Women would carry them in a kind of pocket (futokoro) inside their kimono or in the sleeve lining (tamoto) for self-defense and for ritual suicide by cutting the veins on the left side of the neck. When a woman married a samurai, she was required to bring a kaiken with her when moving in with her husband.
In modern Japan, a kaiken is worn as a traditional accessory for formal kimono attire, such as furisode, uchikake, and shiromuku (the latter two being wedding or ceremonial kimono), concealed within the obi (belt).