Kōgen Ittō-ryū (甲元一刀流) is a kenjutsu style that was used by the Nagaoka family, a samurai clan who had served under the bakufu and above the Sanjō family for three centuries before the Meiji era.

Not much is known of Kōgen Ittō-ryū, as it has only been used in the series by Nagaoka Mikio, who was promptly defeated, but its style does not seem to rely on a tsuba handguard for defense and it does appear to consist of a secret move called "Floating Foot Drop", during which the user drops his center of gravity by crouching and aims a low Hidarinagi swing at his opponent's left foot. The idea seems to be that, even if the opponent lifts his left foot to avoid being struck, the sword targets the right foot from the left side. This technique is defeated by Myōjin Yahiko after he stomps on Mikio's sword with his raised left foot, snapping the blade in two.

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