Siouan languages
In the 19th century, Robert Latham suggested that the Siouan languages are related to the Caddoan and Iroquoian languages. In 1931, Louis Allen presented the first list of systematic correspondences between a set of 25 lexical items in Siouan and Iroquoian. In the 1960s and 1970s, Wallace Chafe further explored the link between Siouan and Caddoan languages. In the 1990s, Marianne Mithun compared the morphology and syntax of all the three families. At present, this Macro-Siouan hypothesis is not considered proved, and the similarities between the three families may instead be due to their protolanguages having been part of a sprachbund.
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).