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.
Yuchi language
Yuchi is classified as a language isolate, because it is not known to be related to any other language. Various linguists have made claims, however, that the language has a distant relationship with the Siouan family: Sapir in 1921 and 1929, Haas in 1951, and 1964, Elmendorf in 1964, Rudus in 1974, and Crawford in 1979.