> g/gh | |- ! colspan="2" | Approximant | * > v | * > l | * > r | * > d | | |} According to Ferlus, * and * are not accepted by all researchers. Ferlus 1992 also had additional phonemes * and *. The fricatives indicated above in parentheses developed as allophones of stop consonants occurring between vowels (i.e. when a minor syllable occurred). These fricatives were not present in Proto-Viet–Muong, as indicated by their absence in Mường, but were evidently present in the later Proto-Vietnamese stage. Subsequent loss of the minor-syllable prefixes phonemicized the fricatives. Ferlus 1992 proposes that originally there were both voiced and voiceless fricatives, corresponding to original voiced or voiceless stops, but Ferlus 2009 appears to have abandoned that hypothesis, suggesting that stops were softened and voiced at approximately the same time, according to the following pattern: > > > > > In Middle Vietnamese, the outcome of these sounds was written with a hooked b (ꞗ), representing a that was still distinct from v (then pronounced ). See below. It is unclear what this sound was. According to Ferlus 1992, in the Archaic Vietnamese period (c. 10th century AD, when Sino-Vietnamese vocabulary was borrowed) it was *, distinct at that time from *. The following initial clusters occurred, with outcomes indicated: *pr, *br, *tr, *dr, *kr, *gr > > > s *pl, *bl > MV bl > Northern gi, Southern tr *kl, *gl > MV tl > tr *ml > MV ml > mnh > nh *kj > gi A large number of words were borrowed from Middle Chinese, forming part of the Sino-Vietnamese vocabulary. These caused the original introduction of the retroflex sounds and (modern s, tr) into the language. Origin of tones Proto-Viet–Muong had no tones to speak of. The tones later developed in some of the daughter languages from distinctions in the initial and final consonants. Vietnamese tones developed as follows: {| class="wikitable" style="text-align:center;" |- ! Register ! Initial consonant ! Smooth ending ! Glottal ending ! Fricative ending |- ! High (first) register | Voiceless | A1 ngang "level" | B1 sắc "sharp" | C1 hỏi "asking" |- ! Low (second) register | Voiced | A2 huyền "deep" | B2 nặng "heavy" | C2 ngã "tumbling" |} Glottal-ending syllables ended with a glottal stop , while fricative-ending syllables ended with or . Both types of syllables