could co-occur with a resonant (e.g. or ). At some point, a tone split occurred, as in many other mainland Southeast Asian languages. Essentially, an allophonic distinction developed in the tones, whereby the tones in syllables with voiced initials were pronounced differently from those with voiceless initials. (Approximately speaking, the voiced allotones were pronounced with additional breathy voice or creaky voice and with lowered pitch. The quality difference predominates in today's northern varieties, e.g. in Hanoi, while in the southern varieties the pitch difference predominates, as in Ho Chi Minh City.) Subsequent to this, the plain-voiced stops became voiceless and the allotones became new phonemic tones. Note that the implosive stops were unaffected, and in fact developed tonally as if they were unvoiced. (This behavior is common to all East Asian languages with implosive stops.) As noted above, Proto-Viet–Muong had sesquisyllabic words with an initial minor syllable (in addition to, and independent of, initial clusters in the main syllable). When a minor syllable occurred, the main syllable's initial consonant was intervocalic and as a result suffered lenition, becoming a voiced fricative. The minor syllables were eventually lost, but not until the tone split had occurred. As a result, words in modern Vietnamese with voiced fricatives occur in all six tones, and the tonal register reflects the voicing of the minor-syllable prefix and not the voicing of the main-syllable stop in Proto-Viet–Muong that produced the fricative. For similar reasons, words beginning with and occur in both registers. (Thompson 1976 reconstructed voiceless resonants to account for outcomes where resonants occur with a first-register tone, but this is no longer considered necessary, at least by Ferlus.) Old Vietnamese {| class="wikitable" style=text-align:center |+ Old Vietnamese Phonology |- ! colspan="2" | ! Labial ! Alveolar ! Palatal ! Velar ! Glottal |- ! colspan="2" | Nasal | m () | n () | nh () | ng/ngh () | |- ! rowspan="3" | Stop | style="background: #f2f2f2;" | tenuis | b/v () | d/đ () | ch/gi () | c/k/q () | # () |- | style="background: #f2f2f2;" | aspirated | ph () | th () | t/r () | kh () | h () |- | style="background: #f2f2f2;" | Implosive stop | m () | n () | nh () | | |- ! rowspan="1" | Fricative | style="background: #f2f2f2;" | voiced | v () | | d () | | |- ! colspan="2"