In the Kamakura period, the Soryo system (the eldest son system for the succession of the head of the family) was established in the samurai world, and at first Yoriko meant the child born out of wedlock who followed the eldest child ("Kamakura Bakufu Tsuika Ho" - additional laws applied by Kamakura Bakufu).
鎌倉時代の武家社会における惣領制の確立によって初期には総領に従う庶子の事を指した（『鎌倉幕府追加法』）。