The Fujiwara clan successfully introduced the Ritsuryo system (a system of centralized government based on the ritsuryo code) between the years 600 and 700; specifically, one of the Fujiwara clan lines, the Northern House of the Fujiwara clan, established its foundations in Togoku (the Utsunomiya clan, the Oda clan, the Nasu clan, the Koyama clan, the Yuki clan, the Sano clan, the Hiki clan, etc.).
西暦600～700年にかけて律令制の導入に成功した藤原氏、特に藤原北家が東国においてもその基盤を確立した（宇都宮氏、小田氏、那須氏、小山氏、結城氏、佐野氏、比企氏等）。