Western Siouan languages
The Western Siouan languages, also called Siouan proper or simply Siouan, are a large language family native to North America. They are closely related to the Catawban languages, sometimes called Eastern Siouan, and together with them constitute the Siouan (Siouan–Catawban) language family.
Siwu language
Siwu has seven oral and five nasal vowels. Several languages of the region have 8- or 9-vowel systems with some form of vowel harmony. In Siwu, noun class prefixes do not harmonize, but root-internally there are some constraints on vowel co-occurrences, attesting to the earlier presence of a system of cross-height vowel harmony (Ford 1973). Siwu is a tonal language with three level tones on the surface: High, Mid, Low. Functional load of tone is high in the lexicon (minimal pairs) as well as in the grammar (tenses marked by tone).