According to old Imperial House Act implemented before World War II, a person of royalty or new nobility was qualified to marry a man of royalty, and there existed a consuetude that a legitimate wife of the main branch of the Imperial Family who was a future Empress (i.e., the crown princess) should be limited to a female from royalty or the Old Sekke.
戦前の旧皇室典範に於いては皇族男子との結婚資格は皇族と華族に与えられていたが、将来の皇后たるべき皇室嫡流の正室（即ち皇太子妃）となるべき者は、皇族若しくは旧摂家の女子に限るという不文律が存在した。