thumb|300px|right|The Asdang, King Vajiravudh, Prince Chakrabongse, Queen Saowabha, Prince Prajadhipok, and Prince Chudadhut)/" class="wiki">Saovabha branch, circa 1900 (from left to right: Prince Asdang, King Vajiravudh, Prince Chakrabongse, Queen Saowabha, Prince Prajadhipok, and Prince Chudadhut)
The 1924 Palace Law of Succession () governs succession to the Throne of the Kingdom of Thailand, under the ruling House of Chakri. The law was established during the reign of King Vajiravudh to systematically resolve previous succession controversies. The 1997 Constitution of Thailand relied on the law with regards to succession, but the 2006 Interim Constitution made no mention of succession, leaving it to "constitutional practice." The 2007 Constitution again relied on the Palace Law with regards to succession.

The law in historical practice

King Chulalongkorn and his first three queens produced seven sons with the princely rank of Chao Fa (; "sky lord" or "celestial prince"): five by Queen Saovabha Bongsri, including Prince Vajiravudh; one by Queen Savang Vadhana, and one by Queen Sukumala. Overall the King had 97 children, including 76 sons (however of lesser number survived into adulthood) but they were of non-royal wives.

None of King Vajiravudh's four princely brothers by Queen Saovabha produced a royal-blooded male heir. Prince Chudadhut Dharadilok produced an heir with a maid. Prince Chakrabongse Bhuvanath produced an heir with a foreign wife (Mom Katya). Children of foreign wives were barred from the succession in section 11 of the Palace Law.

Furthermore, three of the brothers died, leaving only Prince Prajadhipok Sakdidej. Vajiravudh had only a daughter, and Prajadhipok was childless.

Queen Savang Vadhana had only one surviving son, Prince Mahidol Adulyadej, who in turn had a son Prince Ananda Mahidol, later King Rama VII|, on 20 September 1925 with Mom Sangwalya. Prince Mahidol had another son on 5 December 1927, Prince Bhumibol Adulyadej who is the current King of Thailand.

Queen Sukumala produced only one son, Prince Paribatra Sukhumbhand. Prince Paribatra had one son, Prince Chumbhotpong Paripatra, by a royal wife and another son, Prince Sukhumbhinanda, by a commoner wife.

The law based on the principle of male primogeniture, with first in line being the eldest son of the previous monarch and second in line being the next-oldest son, and so on. The law expressly ruled against women ascending the throne (this clause was abrogated in the 1997 Constitution).

According to the Palace Law, Prince Prajadhipok (second eldest surviving son of the previous monarch, King Chulalongkorn, via the bloodline of Queen Saovabha, the most senior queen) became first in line to the succession. Prince Mahidol became second in line (although older than Prajadhipok, he was son of Queen Savang Vadhana, a more junior queen). Third and fourth in line would be Prince Mahidol's sons: first Prince Ananda Mahidol, then Prince Bhumibol Adulyadej. Last in line would have been Prince Paripatra, although this is unlikely given he has been in exile since 1932.

This line of succession was followed, with Prince Prajadhipok succeeding King Vajiravudh after he died on 26 November 1925 as King Rama VII. The absolute monarchy was overthrown on 24 June 1932, but the new constitution continued to rely on the Palace Law regarding matters of succession. Thus, Prince Ananda Mahidol, the eldest heir of Prince Mahidol succeeded the sonless King Prajadhipok after he abdicated on 2 March 1935. Prince Bhumibol Adulyadej succeeded the unmarried young brother and King Ananda Mahidol after he was assassinated on 9 June 1946.

The law today

The 1997 constitution of Thailand, like most preceding Thai constitutions, continued to rely on the Palace Law with regards to succession. However, the 2006 Interim Constitution does not contain any articles regarding succession, and instead, left it to "constitutional practice." Sections 22 and 23 of the 2007 Constitution dealing with succession again relied on the Palace Law.

The heir apparent to the Thai throne is Prince Vajiralongkorn, eldest and only son of King Bhumibol Adulyadej. Prince Vajiralongkorn has many sons
The Thai constitution was amended in 1974 to allow the Privy Council to appoint a princess as successor to the throne. However this would only occur in the absence of an heir apparent. This amendment is retained in Section 23 of the 1997 "People's Constitution." This effectively signaled Princess Sirindhorn as second in line to the throne, but did not affect Prince Vajiralongkorn's status as heir apparent.

Recent constitutions of Thailand have made the amendment of the Palace Law of Succession the sole prerogative of the reigning King. According to Gothom Arya, noted public intellectual, this allows the reigning king, if he so chooses, to appoint his son or any of his daughters to the Throne.

Possible line of succession

The current line of Succession is:

• H.R.H. Crown Prince Maha Vajiralongkorn (Son and the heir apparent of King Rama Ix, the current king)

• H.R.H. Princess Maha Chakri Sirindhorn (The second daughter of King Rama IX and younger sister of No.1.) She was officially promoted as “Somdet Phra Theprat Ratsuda Chao Fa Maha Chakri Sirindhorn Ratthasima Khunakon Piyachat Sayam Borommaratchakumari” and to be the second in line to the throne on December 5, 1977 since then. Her royal title in Thai language contains the word 'Crown Princess' which is almost equivalent to her brother. But in English language, she is styled 'Her Royal Highness' and is equivalent to the British title of The Princess Royal. The word 'Crown Princess' can not be styled in English, because in worldwide tradition, in case of the Crown Prince was styled in the country, it is restrictly used for the formal wife of Crown Prince only.

• H.R.H Princess Chulabhorn Valailak (Youngest daughter of King Rama IX)