is considered separate from the others, so combinations of consonants from adjacent syllables are never recognised as a cluster. Southern thai has phonotactical constraints that define permissible syllable structure, consonant clusters, and vowel sequences. Original Thai vocabulary introduces only 11 combined consonantal patterns: (กร), (กล), (กว) (ขร,คร), (ขล,คล), (ขว,คว) (ปร), (ปล) (พร), (ผล,พล) (ตร) Finals All plosive sounds are unreleased. Hence, final , , and sounds are pronounced as , , and respectively. * The glottal stop appears at the end when no final follows a short vowel. Vowels The vowels of the Southern Thai are similar to those of Central Thai. They, from front to back and close to open, are given in the following table. The top entry in every cell is the symbol from the International Phonetic Alphabet, the second entry gives the spelling in the Thai alphabet, where a dash (–) indicates the position of the initial consonant after which the vowel is pronounced. A second dash indicates that a final consonant must follow. The vowels each exist in long-short pairs: these are distinct phonemes forming unrelated words in Isan, but usually transliterated the same: เขา (khao) means "he/she", while ขาว (khao) means "white". The long-short pairs are as follows: The basic vowels can be combined into diphthongs. For purposes of determining tone, those marked with an asterisk are sometimes classified as long: Additionally, there are three triphthongs. For purposes of determining tone, those marked with an asterisk are sometimes classified as long: Differences from Central Thai Although of the major regional languages of Thailand, Southern Thai is most similar in lexicon and grammar to Central Thai, the varieties are sufficiently different that mutual intelligibility between the two can be problematic. Southern Thai presents a diglossic situation wherein registers range from the most formal (Standard Central Thai spoken with Southern Thai tones and accent) to the common vernacular (usually a contracted form of Thai expressions and with some amount of loan words from Malay). The Thai language was introduced with Siamese incursions into the Malay Peninsula possibly starting as early as the Sukhothai Kingdom. During this and successive kingdoms, the area in which Southern Thai is spoken was a frontier zone between Thai polities and the Malay Sultanates. Malay vocabulary has been absorbed into the lexicon, as a considerable number of Malay speakers lived in or near Patani polity and interacted with the Thai speakers through