used to demarcate tone. As codas are forbidden in Nuosu there is no ambiguity. Origin and development André-Georges Haudricourt established that Vietnamese tone originated in earlier consonantal contrasts and suggested similar mechanisms for Chinese. It is now widely held that Old Chinese did not have phonemically contrastive tone. The historical origin of tone is called tonogenesis, a term coined by James Matisoff. Tone as an areal feature Tone is sometimes an areal rather than a phylogenetic feature. That is to say, a language may acquire tones through bilingualism if influential neighbouring languages are tonal or if speakers of a tonal language shift to the language in question and bring their tones with them. The process is referred to as contact-induced tonogenesis by linguists. In other cases, tone may arise spontaneously and surprisingly fast: the dialect of Cherokee in Oklahoma has tone, but the dialect in North Carolina does not although they were separated only in 1838. In world languages Tone arose in the Athabascan languages at least twice, in a patchwork of two systems. In some languages, such as Navajo, syllables with glottalized consonants (including glottal stops) in the syllable coda developed low tones, whereas in others, such as Slavey, they developed high tones, so that the two tonal systems are almost mirror images of each other. Syllables without glottalized codas developed the opposite tone. For example, high tone in Navajo and low tone in Slavey are due to contrast with the tone triggered by the glottalization. Other Athabascan languages, namely those in western Alaska (such as Koyukon) and the Pacific coast (such as Hupa), did not develop tone. Thus, the Proto-Athabascan word ('water') is toneless in Hupa, high-tone in Navajo, and low-tone tù in Slavey; while Proto-Athabascan ('knee') is toneless in Hupa, low-tone in Navajo, and high-tone in Slavey. provides a phonetic explanation for the opposite development of tone based on the two different ways of producing glottalized consonants with either tense voice on the preceding vowel, which tends to produce a high F0, or creaky voice, which tends to produce a low F0. Languages with "stiff" glottalized consonants and tense voice developed high tone on the preceding vowel and those with "slack" glottalized consonants with creaky voice developed low tone. The Bantu languages also have "mirror" tone systems in which the languages in the northwest corner of the Bantu area have the opposite tones of other Bantu languages.