The Palace Law of Succession, Buddhist Era 2467 (1924) (; ) governs succession to the Throne of the Kingdom of Thailand, under the ruling House of Chakri. Succession matters prior to the end of absolute monarchy in 1932 could be contentious, especially during the Ayutthaya period from the 14th to 18th centuries. In 1924, King Vajiravudh (Rama VI) attempted to clarify the succession process by laying down the Palace Law of Succession. It was promulgated and came into effect in November 1924 as, in part, an attempt to eliminate the vagueness relating to succession within the Thai monarchical regime and to systematically resolve previous controversies. In 1932, after Siam became a constitutional monarchy, various amendments relating to succession were introduced. 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. The preamble of the 2014 interim constitution of Thailand abrogated the 2007 Constitution, with the exception of chapter 2, concerning the monarchy and the succession.
Background
The original Palace Law of A.D. 1360 relating to succession since Ayutthaya Kingdom period (1351–1767) did not lay out a clear system for determining a successor upon the death of a king. Rather, it provided a frame of reference from which the next king could be chosen. Typically, the new king would be either the late king's son born of a major queen or consort (; ), or one of his brothers. The Law also provided rules by which someone who was neither a son nor a brother of the deceased king could accede to the throne, should the situation or circumstances require it.
However, the Palace Law was not always followed and did not ensure smooth successions. At least one third of Ayutthaya's royal successions involved bloodshed. Indeed, the history of the kingdom at that time is a chronicle of frequent usurpations and of ambitious men thwarting the final wishes of recently departed kings. Historian David K. Wyatt observed that "virtually all successions to the throne of Ayutthaya in the seventeenth and eighteenth centuries were, at the least, irregular, and in many cases either disguised or real usurpations."
King Taksin (reigned 1767–1782) established himself as King of Siam by right of conquest. He, however, alienated the ecclesiastical support to secure the legitimacy of his reign, and was overthrown in a palace coup. His chief military commander, then on an expedition to enforce recognition of his friend and king by the current ruler of Cambodia, hastened to the rescue. When apprised of the full seriousness of situation, however, he concurred in the overthrow and founded the Royal House of Chakri as King Buddha Yodfa Chulaloke (later styled Rama I, reigned 1782–1809.) He next established the office of the Supreme Patriarch of Thailand. Then, following the precedent of the newly crowned King Wareru of the 13th century, the newly crowned king of Siam commissioned scholars to collect and revise laws from the Ayutthaya era. The Code of Wareru is reflected in successive codes, but the Siamese went beyond it to establish a true code of laws. The Palace Law was a constituent part of this new legal compendium called The Three Seals Code, which was needed, Rama I said, because old laws were often misinterpreted and this led to injustice. Succession under the Three Seals Code took into account the potential of the next king to be, as prescribed by ancient Buddhist texts, a dhammaraja (righteous king). This was manifested in his upholding of the Ten Virtues of Kingship. Lineage, however, still played a very important role.