[…] they [ the tones] may be melodic, i.e. opposed as rising to falling, as in Lithuanian, as glottalized to non-glottalized, as in Danish, as a simple to complex, as in Swedish […] If melodic, the tones may be all on the same register as in Mandarin Chinese or in the languages just quoted; or they may be in different registers as in Vietnamese, where it is not enough to say that a tone is rising or glottalized, but where one has to state whether it is high rising or low rising, high glottalized or low glottalized.
- Martinet (1962), a pag.85