After the era of Emperor Ninmyo, Imperial succession of the direct Imperial line continued almost smoothly for example; Emperor Montoku - Emperor Seiwa - Emperor Yozei, later Emperor Yozei caused a serious incident in the Palace, (there was a theory about him accidentally killing someone) after he was forced to abdicated from the throne, and again there was a crisis of the discontinuity of Imperial succession of the direct Imperial line.
仁明の後、文徳天皇－清和天皇－陽成天皇と直系による皇位継承がほぼ順調に行われたが、陽成が内裏で重大事件（誤って殺人を犯したとする説が有力）を起こし退位に追い込まれてしまい、再び直系継承の下で皇統断絶の危機が訪れることになった。