Western linguists generally classify Tai Dam and Tai Dón as two separate languages (the Vietnamese government considers them to be two dialects of the same language). It is not known whether Jinping Dai should be classified as a separate language or as a dialect of one of the other two languages.
The three languages differ with respect to tone systems, but these differences are not represented in the writing.
The phonetic inventories of Tai Dón and, it is believed, Jinping Dai, contain four syllable-initial aspirated consonants which Tai Dam does not. This difference is accounted for in the Tai Viet character repertoire as encoded in The Unicode Standard. High and low form characters for /ph/, /tɕh/ and /kh/ were added for the purpose of representing these additional Tai Dón consonants. (The fourth additional consonant, /khw/ can be written using the labialization marker discussed in 'behaviour'.) Even using these additional characters however, Tai Dón can only be written using the orthographic conventions of Son La province. Additional characters would need to be encoded to enable it to be written with Lai Chau conventions.
The Tai Dam phonetic inventory contains three vowel glides, /iə/, /ɨə/ and /uə/, which in Tai Dón and Jinping Dai consistently change to /e/, /ə/ and /o/ respectively. Accordingly, in the writing, the symbols used for /iə/ and /uə/ in Tai Dam are used to represent /e/ and /o/.
There is considerable variation in orthographic conventions between speakers of the Tai Dam, Tai Dón and Jinping Dai languages, as well as within the Tai Dón community.
Jim Brase, 'Writing Tai Don - additional characters needed for the Tai Viet script' 2008