Macro-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, the Macro-Siouan hypothesis based on relations among Siouan, Caddoan, and Iroquoian is not universally accepted as proven.
Assiniboine language
Some scholars argue that the D-N-L classification system may not be totally accurate due to the non-rigidness of the substitution form. Siouan Indians live on an expansive continuum such that the distinction between different languages does not manifest in a rigid, clear-cutting criterion. Historically, linguists have debated on Yankton-Yanktonai languages and their proper positions into the D-N-L classification system, but the coexistence of /d/ and /n/ phonemes made such classification doubtful. This example of lexical difference between the languages of the Siouan group illustrates another possible distinction besides the D-N-L variations.
The phonemic inventory has 27 consonants, which includes aspirated, plain, and ejective stops. In addition to this, it has five oral vowels and three nasal vowels. It is a structure-preserving language. Assiniboine has no definite or indefinite articles, no nominal case system, and no verbal tense marking. Clauses unmarked are "realized," while clauses marked as "potential" by means of verbal enclitic, which is successful in producing a future/non-future distinction. The verbal system is split into active and stative (split-intransitive). The active object pronominal affixes coincide with the stative verbs of the subject pronominal affixes.
The stops (and affricates) of Assiniboine are often described as voiced rather than plain, due to intervocalic voicing rules which result in surface voiced forms. Nonetheless, these should be analyzed as plain.