About Mongkut
Mongkut, posthumously honoured as King Mongkut the Great was the king of Siam from the Chakri dynasty, titled Rama IV. He reigned from 1851 until his death in 1868. Although the term "Siam" had long been used by foreigners to refer to the kingdom, the title "King of Siam" came to be formally adopted during his reign, particularly in official and diplomatic contexts.
Biography source:
Wikipedia
