Siouan languages
Authors who call the entire family "Siouan" distinguish the two branches as "Western Siouan" and "Eastern Siouan" or as "Siouan-proper" and "Catawban". Others restrict the name "Siouan" to the western branch and use the name "Siouan–Catawban" for the entire family. Generally, however, the name "Siouan" is used without distinction.
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).