The Yamaguchi clan, who had ruled the Ushiku Domain for twelve generations, was allegedly descended from the distinguished Ouchi clan, arguably the strongest specimen of Saigoku Daimyo (Japanese territorial lords in western Japan) of the Sengoku period (period of warring states).
牛久藩を12代にわたって支配した山口氏は、戦国時代(日本)の西国大名では最強と呼ばれた名門・大内氏の系統と言われている。