http://www.aikidojournal.com/?id=1895
We have, however, proof that Ueshiba learned Yagyu Shinkage-ryu with some degree of depth. This proof lies in the sword method of Hikitsuchi Michio. Hikitsuchi taught three sword forms, called Sho (matsu — pine), Chiku (take — bamboo), and Bai (ume — plum). According to Meik Skoss, who trained under Hikitsuchi, "Sho" embodies Irimi — the triangle; "Chiku" embodies Tenkan — the circle: and "Bai" embodies Osae — the square. Fascinatingly, these three kumitachi forms are modifications of forms from Yagyu Shinkage-ryu: "Sho" is Kaboku, #4 from Kuka no Tachi; "Chiku" is Settetsu, #2 from Sangakuen no Tachi; and "Bai" is Ozume, #7 from Kuka no Tachi. In essence, then, not only did Ueshiba learn a good deal of the Yagyu Shinkage-ryu curriculum, he abstracted out three different forms as a: the embodiment of what were, to him, the three fundamental principals of his art, and b. creating a curriculum, using these forms as "containers," specifically tailored to Hikitsuchi Michio.