Historically, the Imperial Throne was determined by considering choyo no jo (a uniform order between senior and junior) and capabilities of children and influences of maternal relatives, and the first son could not always become crown prince.
歴史的に皇位は、長幼の序を重んじつつ、本人の能力や外戚の勢力を考慮して決定され、長男であれば必ず皇太子になれるとは限らなかった。