Tone marks as vowel diacritics in two scripts: repurposing tone marks for non-tonal phenomena in Cado and other Southeast Asian languages

ABSTRACT Many scripts in Southeast Asia have developed ways of marking tone. When adapting such scripts to write non-tonal languages, language communities often find other uses for these symbols. The case of Cado, an Austroasiatic language of the Katuic sub-branch in Vietnam and Laos, is particularly striking in that they use tone marks as vowel diacritics in two different scripts. Because Cado speakers live on both sides of the Vietnamese-Lao border, they have chosen to use two separate writing systems based on their respective national languages. This paper presents preliminary orthographies for Cado in the Roman and Lao scripts, based on the Vietnamese and Lao orthographies. Cado has no phonation contrast, but it does retain vowel length contrast. Both Cado orthographies adapt tone marks from the Vietnamese and Lao orthographies to distinguish either vowel length or vowel quality respectively. The phonetic motivation for this cross-categorical use of tone marks is discussed, and examples of other orthographies in Southeast Asia that adapt tone marks and other symbols beyond their traditional phonetic category are given and compared with Cado.