entire Kra–Dai family, spoken mainly in China, Vietnam, Thailand, and Laos, and including Thai and Lao, is tonal. The entire Hmong–Mien family is highly tonal. Many Afroasiatic languages in the Chadic and Omotic branches have tone systems, including Hausa. The vast majority of Niger–Congo languages, such as Ewe, Igbo, Lingala, Maninka, Yoruba, and the Zulu, have tone systems. The Kru languages and Southern Mande languages have the most complex. Notable non-tonal Niger–Congo languages are Swahili, Fula, and Wolof. All Nilotic languages such as the Dinka language, the Maa languages, the Luo languages and Kalenjin languages have tone systems. All Khoisan languages in southern Africa have tone systems; some languages like Sandawe have tone systems like that of Cantonese. Slightly more than half of the Athabaskan languages, such as Navajo, have tone systems (languages in California and Oregon, and a few in Alaska, excluded). The Athabaskan tone languages fall into two "mirror image" groups. That is, a word which has a high tone in one language will have a cognate with a low tone in another, and vice versa. Iroquoian languages like Mohawk commonly have tone; the Cherokee language has the most extensive tonal inventory, with six tones, of which four are contours. Here the correlation between contour tone and simple syllable structures is clearly shown; Cherokee phonotactics permit only syllables of the structure (s)(C)V. All Oto-Manguean languages are tonal. In some cases, as with Mixtec, tone system variations between dialects are sufficiently great to cause mutual unintelligibility. The Ticuna language of the western Amazon is strongly tonal. Various Arawakan languages have relatively basic tone systems. Many languages of New Guinea like Siane possess register tone systems. Some Indo-European languages (notably Swedish, Norwegian, Latvian, and Serbo-Croatian) as well as others possess what is termed pitch accent, where only the stressed syllable of a word can have different contour tones; these are not always considered to be cases of tone language. Some European-based creole languages, such as Krio, Saramaccan and Papiamento, have tone from their African substratum languages. In some cases, it is difficult to determine whether a language is tonal. For example, the Ket language of Siberia has been described as having up to eight tones by some investigators, as having four tones by others, but by some as having no tone at all. In cases such as these, the classification of a language as tonal may depend on the researcher's