Emperor Keitai, who was from the collateral line, seems to have tried to prove his legitimacy as Emperor by marrying to Princess Tashiraka, who was the younger sister of late Emperor with legitimate and direct bloodline, taking the form of Irimuko (man who takes his wife's premarital family name).
これは傍系である継体天皇は、先代天皇の妹で正当な血筋を持つ直系の手白香皇女を皇后にする事により、一種の入り婿という形で正当性を誇示したと考えられている。