Thailand is one of a few monarchies (like Denmark and Sweden) that have a separate anthem for the royal family, as opposed to the national anthem for the citizens. The Thai royal anthem is performed during state occasions and public meetings, as well as when a high-ranking member of the royal family is present for a function.
The royal anthem also served as the third national anthem of Siam (as Thailand was then known) until the 1932 coup. It is unclear who composed the music of the anthem; some sources give the Russian Pyotr Schurovsky, some sources list a Dutch composer named Hevutzen, and others a traditional Thai melody. The lyrics were written by Prince Narisara Nuvadtivongs, the brother of the king, and were revised by King Rama VI in 1913 and officially adopted as the royal (and national) anthem at that time.
Special thanks to: Artakorn Jarusriwanna for providing me with the sheet music.