The head of the Mito family was regarded as the assistant to Seiitaishogun, so the family was excused from sankinkotai (a system under which feudal lords were required to spend every other year in residence in Edo) and remained in Edo all the time.
水戸家当主は征夷大将軍の補佐役であるとされ、参勤交代せず江戸定府の例であった。