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.
Duan language
Duan, Doan, or Halang Doan, is a language spoken by more than four thousand people on either side of the Laotian–Vietnamese border. There are some 2,346 speakers in Attopu Province, Laos, and another couple of thousand in Kon Tum Province, Vietnam. It is too poorly known to classify completely and may be mutually intelligible with Takua, Kayong, Halang, and Rengao. Might be a part of the Xơ Ɖăng ethnic group.