Phra

Thai royal and noble titles are the royal and noble styles indicating relationship to the king which were introduced by King Trailokanat (reigned 1448–1488). The system is rooted in the Thai language equivalent of feudalism, Sakdina (: "power over fields"). Borne of a polygamous royal tradition in which monarchs and noblemen often had dozens of children, such royal and noble titulary is similar to Western concepts of peerage. A title of honor may not be confined to the beginning or the end of the name, but may be split across the name. Those with titles ceased to be known by personal names, and were referred to by the awarded (or similar) title. The King could bestow a title on anyone, although such promotion was personal and the person's children would not normally benefit from it. The complex and nuanced distinctions in rank of the titles are largely uncaptured by their glosses into Western-style terms. Bestowing noble titles ended after the Siamese revolution of 1932, and the adoption of monogamy resulted in far fewer royal consorts and issue who could receive these styles. As a result, the old titles are obscure even to most Thais.

There are two styles which can be used for a king in ordinary speech, depending on whether he has been crowned:

Traditionally, titles of royal wives depended on their birth titles and royal favour; only princesses of high birth (Chao Fa, Phra Ong Chao and Mom Chao Ying) assuming titles higher than Chao Chom. There were no clear rules about the hierarchy of titles above Chao Chom until the time of King Mongkut, and titles changed over successive reigns. The rule about commoners also seems to be evolving, and it appears that there are no more restrictions on a commoner from becoming queen. Most of the titles below are from King Vajiravudh's 1924 enactment of the Succession Law.

Holders of these titles are still considered royal, since they are (at most) two generations removed from a king. Nai Luang (ในหลวง) is an epithet for a king. Children of a king are called Luk Luang ( "royal children"), and grandchildren of a king are called Laan Luang ( หลวง "royal grandchildren"). In English, they are normally called "prince" or "princess". Special forms are used when one wishes to address them, although the language is less elaborate than when speaking to the king or the queen. A male Luk Luang who does not accede to the throne would assume a new royal surname, normally reflecting his birth name (as opposed to an honorific given later). The surname can be used by his wife if she is a commoner by birth, possibly with Na Ayudhya added if she has no noble title. It is otherwise not normally used until his children (or grandchildren) first hold the title of Mom Chao, when the surname will first appear in their names.

...
Wikipedia