Lao and (Central) Thai are two closely related languages of the Southwestern branch of Tai languages. Lao falls within the Lao-Phuthai group of Southwestern Tai languages and Thai within the Chiang Saen language group. Lao (including Isan) and Thai, although they occupy separate groups, are mutually intelligible and were pushed closer through contact and Khmer influence, but all Southwestern Tai languages are mutually intelligible to some degree.Paul, L. M., Simons, G. F. and Fennig, C. D. (eds.). 2013. Ethnologue: Languages of the World, Seventeenth edition. Dallas, Texas: SIL International. Retrieved from http://www.ethnologue.com Isan refers to the local development of the Lao language in Thailand, as it diverged in isolation from Laos, under Thai influence. The Isan language is still referred to as Lao by native speakers.Keyes, Charles F. (1966). "Ethnic Identity and Loyalty of Villagers in Northeastern Thailand". Asian Survey. Spoken Lao is mutually intelligible with Thai and Isan to such a degree that their speakers are able to effectively communicate with one another speaking their respective languages. These languages are written with slightly different scripts, the Lao script and Thai script, but are linguistically similar and effectively form a dialect continuum.

Although Thai and Lao (including Isan) are mutually intelligible, Thai speakers without previous exposure to the Isan language encounter several difficulties parsing the spoken language. Isan, written according to Thai etymological spelling, is fairly legible to Thai as the two languages share more than eighty percent cognate vocabulary, similar to the relationship between Spanish and Portuguese as changes in the meanings of terms, retention of archaisms, slightly different grammar and some vocabulary differences blur the close relationship.Hesse-Swain, C. (2011). "Speaking in Thai, Dreaming in Isan: Popular Thai Television and Emerging Identities of Lao Isan Youth Living in Northeast Thailand" (Master's thesis, Edith Cowan University) (pp. 1–266). Perth, Western Australia. The relationship is asymmetric, with Isan speakers able to understand spoken and written Thai quite well due to its mandatory use in school and the popularity of Thai media and participation in Thai society, but many Isan students suffer the shock of switching from the Isan language of the home to the Central Thai-only primary school.

False friends Many Lao terms are very similar to words that are profane, vulgar or insulting in the Thai language, features that are much deprecated. Lao uses ອີ່ ( and ອ້າຍ/archaic ອ້າຽ (), to refer to young girls and slightly older boys, respectively. In Thai, the similarly sounding อี, i () and ไอ้, ai () are often prefixed before a woman's or man's name, respectively, or alone or in phrases that are considered extremely vulgar and insulting. These taboo expressions such as อีตัว "i tua", "whore" () and ไอ้บ้า, "ai ba", "weirdo" ().

+ False CognatesIsanLaoIPAUsageThaiIPAUsage บัก, bak ບັກ, bak Used alone or prefixed before a man's name, only used when addressing a man of equal or lower socio-economic status and/or age. บัก, bak Alone, refers to a "penis" or in the expression บักโกรก, bak krok, or an unflattering way to refer to someone as "skinny". หำน้อย, ham noi ຫຳນ້ອຍ/archaic ຫຳນ້ຽ, ham noi Although ham has the meaning of "testicles", the phrase bak ham noi is used to refer to a small boy. Bak ham by itself is used to refer to a "young man". หำน้อย, ham noi This would sound similar to saying "small testicles" in Thai, and would be a rather crude expression. Bak ham is instead ชายหนุ่ม, chai num () and bak ham noi is instead เด็กหนุ่ม, dek num () when referring to "young man" and "young boy", respectively, in Thai. หมู่, mu ໝູ່, mou Mu is used to refer to a group of things or people, such as ໝູ່ເຮົາ/ຫມູ່ເຮົາ, mou hao (), or "all of us" or "we all". Not to be confused for ໝູ/ຫມູ mou (), pig. พวก, phuak The Isan word หมู่ sounds like the Thai word หมู (), 'pig', in most varieties of Isan. To refer to groups of people, the equivalent expression is พวก, phuak (), i.e., พวกเรา, phuak rao () for "we all" or "all of us". Use of mu to indicate a group would make the phrase sound like "we pigs". ควาย, khwai ຄວາຍ/archaic ຄວາຽ, khouay Isan vowel combinations with the semi-vowel "ວ" are shorted, so would sounds more like it were written as ควย. ควาย, khwai Khwai as pronounced in Isan is similar to the Thai word ควย, khuai (), which is another vulgar, slang word for "penis".

Phonological differences

Thai and Lao share a similar phonology, being closely related languages, however, several developments occurred in Lao that clearly distinguish them. Tone, including patterns and quality, is the largest contributing factor and varies widely between varieties of Lao, but together they share splits quite distinct to Ayutthaya dialect (Standard Thai) and other Central Thai dialects. There are also several key sound changes that occurred in the Lao language that differentiates it from Thai.

Consonantal differences Lao lacks the /r/ of formal Thai, replacing it with /h/ or /l/, as well as /tɕʰ/, which is replaced by /s/. Lao also has the consonant sounds /ɲ/ and /ʋ/, which are absent in Thai. Aside from these differences, the consonantal inventory is mostly shared between the two languages.

C1C2 > C1 Unlike Thai, the only consonant clusters that traditionally occur is C/w/, limited in Lao to /kw/ and /kʰw/ but only in certain environments as the /w/ is assimilated into a diphthongization process before the vowels /aː/, /am/, /aːj/ and /a/ thus limiting their occurrence. For example, Isan kwang (, kouang, ) is pronounced *kuang (*, *) but kwaen as in kwaen ban (, khoèn ban, ), 'to feel at home', has a vowel that does not trigger the diphthongization. The consonant clusters of Proto-Tai had mostly merged in Proto-Southwestern Tai, but clusters were re-introduced with Khmer, Sanskrit, Pali and European loan words, particularly C/l/ and C/r/. Lao simplified the clusters to the first element, but sporadically maintained its orthographic representation as late as the early twentieth century although their pronunciation was simplified much earlier. This was likely an influence of Thai.Davis, G. W. (2015). "The Story of Lao r: Filling in the Gaps". The Journal of Lao Studies, 2(2015), pp. 97–109.

In some instances, some loan words are sometimes pronounced with clusters by very erudite speakers in formal contexts or in the speech of Isan youth that is very Thaified, otherwise the simplified pronunciation is more common. Lao speakers, especially erudite speakers may write and pronounce prôkram (, ), via French (), and maitri (, ) from Sanskrit maitri (, ) are common, more often than not, they exist as pôkam (, ) and maiti (, ), respectively. Similarly, Isan speakers always write and sometimes pronounce, in 'Thai fashion', maitri (, ) and prokraem (, ), via English 'programme' or 'program' (US), but most speakers reduce it to and , respectively, in normal speech.

+ Reduction of consonant clusters in Lao Thai Isan Lao Thai Isan Lao Thai Isan Lao ก /k/ ก /k/ ກ /k/ ค /kʰ/ ค /kʰ/ ຄ /kʰ/ ผ /pʰ/ ผ /pʰ/ ຜ /pʰ/ กร /kr/ กร คร /kʰr/ คร ผล /pʰl/ ผล กล /kl/ กล คล /kʰl/ คล พ /pʰ/ พ /pʰ/ ພ /pʰ/ ข /kʰ/ ข /kʰ/ ຂ /kʰ/ ต /t/ ต /t/ ຕ /t/ พร /pʰr/ พร ขร /kʰr/ ขร ตร /tr/ ตร พล /pʰl/ พล ขล /kʰl/ ขล ป /p/ ป /p/ ປ /p/ ปร /pr/ ปร ปล /pl/ ปล + Examples of cosonant cluster reduction in Lao Thai Isan Lao Gloss เพลงphleng /pʰlēːŋ/ เพลงphleng /pʰêːŋ/ ເພງphéng /pʰéːŋ/ 'song' ขลุ่ยkhlui /kʰlùj/ ขลุ่ยkhlui /kʰūj/ ຂຸ່ຍkhouay /kʰūj/ 'flute' กลางklang /klāːŋ/ กลางklang /kàːŋ/ ກາງkang /kàːŋ/ 'centre''middle' ครอบครัวkhrop khrua /kʰrɔ̂ːp kʰrūa/ ครอบครัวkhrop khrua /kʰɔ̂ːp kʰûa/ ຄອບຄົວkhop khoua /kʰɔ̑ːp kʰúːə/ 'family'

/r/ > /h/ Proto-Southwestern Tai initial voiced alveolar trill remained [r] in Thai, although it is sometimes pronounced /l/ in informal environments, whereas Lao changed the sound to the voiceless glottal fricative /h/ in these environments. The sound change likely occurred in the mid-sixteenth century as the Tai Noi orthography after that period has the letter Lao letter '' , which was a variant of '' /r/ used to record the sound change. The change also included numerous small words of Khmer origin such as hian , ), 'to learn', which is rian (, ) in Thai, from Khmer riĕn (, ).

+ Proto-Southwestern /r/ to /h/ in Lao PSWT Thai Isan Lao Gloss *rim rim him him 'edge', 'rim', 'shore' (Lao/Isan) *rak rak hak hak 'to love' *rɔn ron hon hon 'to be hot' *rɯə ruea huea hua 'boat'

/r/ > /l/ The shift of Proto-Southwestern Tai */r/ to /h/ in Lao was inconsistent, with some factors that prevented the transition. Instead, these situations led to the shift of /r/ to the alveolar lateral approximant /l/, similar as to what occurs in informal, casual Thai. Polysyllabic loan words from Khmer as well as Indic sources such as Khmer and Pali may have seemed too 'foreign' compared to the monosyllabic loan words that may have been regarded as native, somewhat similar to English 'beef', ultimately from French boeuf but fully anglicized in spelling and pronunciation, versus more evidently French loan words such as crème anglaise, which retains a more French-like pronunciation. Thai speakers sometimes use /l/ in place of /r/ in relaxed, basilectal varieties but this is deprecated in formal speech.

rasa ( raxa, ) from Sanskrit rājā (, ), 'king', cf. Thai racha (, ), 'king' raka (, ), 'price' from Sanskrit rāka (, ), 'wealth', cf. Thai raka (, ) charoen ( chaluen, ), 'prosperity', from Khmer camraeum (, ), cf. Thai charoen (, ) rabam (, , 'traditional dance', from Khmer rôbam (, ), cf. Thai rabam (, )

Lao and Thai both have digraphs, or in the case of Lao ligatures, that consist of a silent /h/ that was historically pronounced at some ancient stage of both languages, but now serves as a mark of tone, shifting the sound to a high-class consonant for figuring out tone. The /h/ may have prevented the assimilation of these words to /h/, as these end up as /l/ in Lao. Similarly, this may have also prevented /r/ to /h/ in Khmer loan words where it begins the second syllable.

rue ( lu, ) versus rue (, ), 'or' (conjunction) lio (, versus ri (, ), 'to squint' (one's eyes) kamlai (, ), 'profit', from Khmer kâmrai (, ), cf. Thai kamrai (, ) samrap ( samlap, ), 'for' (the purpose of, to be used as, intended as), from Khmer sâmrap (, ), cf. Thai samrap (, )

There are a handful of words where the expected conversion to /h/ did not take place, thus yielding /l/. In some cases, even in the Lao of Laos, this can be seen as historic Siamese influence, but it also may have been conservative retentions of /r/ in some words that resisted this change. For example, Isan has both hap (, , ) and lap (, , ), both of which mean 'to receive' and are cognates to Thai rap (, ), and the lap variety in Isan and parts of Laos, especially the south, may be due to Thai contact. In other cases, it is because the words are recent loans from Thai or other languages. In Isan, younger speakers often use /l/ in place of /h/ due to language shift.

ro (, ), 'to wait, to wait for', cf. Thai ro (, ) rot ( lôt, ), 'car' or 'vehicle', cf. Thai rot (, ) lam (, ), 'to dance', cf. Thai ram (, ) rom ( rôm, ), 'Rome', cf. Thai rom (, ) rangkai (, ) (Isan youth), traditionally hangkai (, , ), 'body' (anatomic), cf. Thai rangkai (, )

/tɕʰ/ > /s/