Until relatively recently, the position of heir apparent or heir presumptive in the Siamese royal hierarchy. The king would appoint a uparat, usually a brother, son, or uncle.
When one of the better-known Siamese kings, Mongkut, was crowned in 1851, he elevated his brother Chudamani to uparat to reign jointly with him. He may have done so to neutralize his powerful brother (and his brother's small army) by holding open the possibility of immediate power, as well as an enhanced chance at succession to the throne. Perhaps because this man was a brother and not a son, farang were confused about his status, and usually referred to him as the second king. Relations between the brothers were strained, in part because many farang preferred the more even-tempered and Europeanized Chudamani to the volatile Mongkut.
Mongkut's son Chulalongkorn in turn appointed Chudamani's son, George Washington, to be his uparat. However, when George died Chulalongkorn, aware that Europeans were confused about the institution of uparat, elevated one of his own sons to the position of crown prince, a rank more consonant with European tradition and thus more comprehensible to farang. Since then, Siam, now Thailand, has had no more uparat, only crown princes and, recently, a crown princess.