Ikan (traditional formal court dress) was originally a costume for night duty (called tonoiginu) in the Imperial Court, however, it started to take root as a working costume in the Imperial Court, then sokutai became to be worn as a formal costume in ceremonies.
衣冠は、本来、宮中に於ける宿直用の装束（とのいぎぬ）であったが、宮中での勤務服として定着するにつれ、束帯は儀式に用いる儀礼的な服となった。