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.
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