There are no clauses that state qualifications to become the empress in the Taiho Code but since it states that 'Kisaki,' who was one rank below the empress, needed to be 'a princess with a rank above 'Shihon,' (the fourth rank of Imperial Prince's rank) by definition, people were resigned to think that the empress had to be a princess.
大宝律令に皇后になれる資格を規定した条文はないが、皇后より一段下位の妻である妃の資格が「四品以上の内親王」と規定されていることから、皇后も当然内親王でなければなれないものと観念されていたとする考え方がある。