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.
Sioux language
Sioux has three major regional varieties, with other sub-varieties:Yankton-Yanktonai (Western Dakota) stands between Santee-Sisseton (Eastern Dakota) and Lakota within the dialect continuum. It is phonetically closer to Santee-Sisseton but lexically and grammatically, it is much closer to Lakota. For this reason Lakota and Western Dakota are much more mutually intelligible than each is with Eastern Dakota. The assumed extent of mutual intelligibility is usually overestimated by speakers of the language. While Lakota and Yankton-Yanktonai speakers understand each other to a great extent, they each find it difficult to follow Santee-Sisseton speakers.