Kaiken is one of the styles of koshirae as you know. It can be called as "futokoro katana" sometimes. "Futokoro" means in the cloths.
Kaiken is used to take under the cloths. It is not used for display on the waist. So the whole design has smooth outline, and no kozuka slot, no kaeri-zuno. Usually it has no metal fittings, and some parts are made of buffalo horn. Kurikata is omitted sometimes, and so is menuki.
The blade length is not a matter of it. Usually shorter than 30cm, because of convenience for putting in the cloths. This is used by both men and women.
A kaiken with a Sanskrit letter on the head. It is an initial of a Buddha.

A kaiken with a family mark.

A kaiken with gold dragon.

Usually the word "Kaiken" brings us the image of women's one. For women in samurai family, kaiken is necessary as same as Dai-sho sword for men.  Women take it within a bag and put into the sash. The tassel and the cloth of the bag is shown at the breast as fashion of ladies.
Examples of Kaiken in casual form, from the TV program "Atsuhime" by NHK.
Even now, brides put kaiken into their sash at the wedding. But, the kaiken is just a wood stick, the bag and the tassel shows the remembrance of samurai age.
A bride for example. The tassel and the bag are pure white, and so is the wedding dress under the red over dress.
In such ladies kaiken, blade length is not so long as men's. Some people say around 6 sun (18cm) is common. Probably the length is good as a part of dressing. When we make kaiken blades for brides, the length are settled around 20cm.

Kaiken is not made for display, it is in the cloth or in the bag. But we find such kaikens those have decorations on them. They can not be seen by other people. It is for private pleasure. Sometimes the motif means policy of the owner, or wishes of parents who ordered it for their daughter. Kaiken can be a spiritual amulet, as well as an actual dagger.

This has a Hebraic letter on its head. That is not found in old swords, but the spirit is the same.

Some people claims about the blade of kaiken, that the hamon should be suguha and yakizume (straight pattern and no turn back). A straight hamon pattern suggests a quiet wife, and no turn back suggests the bride never return to the parents (no divorce). I think that is a kind of superstitions made recently. I have not found such blades in kaiken koshirae from Edo period.

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