It is believed that the party was based in Yokoyama, Tama County, Musashi Province (the present Moto-Yokoyama Town, Hachioji City, Tokyo) and gave themselves the family name of Yokoyama.
武蔵国多摩郡横山（現・東京都八王子市元横山町）を本拠として横山姓を称したとされる。