The redivision of tone-bearing vowels into separate vowel phonemes and tone phonemes may be considered a special case of this rephonemicization. This is done in ‘tone languages’, where the sequences of tones do not show a limited number of contours […]. However, the division of, say, high-pitched [à] into /a/ and /'/ (high tone), and [é] into /e/ and /'/, and so on, is based not on any exceptional distribution of a particular tone-bearing vowels in general. Rather, it is based on the convenience of separately describing the vowels of a sequence and its tones […].
- Harris (1951), a pag.92