King Bhumibol Adulyadej officially appointed Prince Vajiralongkorn, his eldest and only son, as Crown Prince and successor on 28 December 1972. The Thai constitution was amended in 1974 to allow the Privy Council to appoint a princess as successor to the throne, if the monarch had not previously appointed a successor. This change made for the possibility of an eventual female succession in the absence of male heir (something not hitherto allowed for). These rights of succession of a female are strictly defined and apply only to the daughters of a king, and may only come into effect following the approval of the legislative assembly. However this would definitely occur in the absence of a royally appointed successor. This amendment is retained in Section 23 of the 1997 "People's Constitution" and 2007 Constitution, and Section 21 of the 2017 Constitution.