VoltaâÂÂCongo is a major branch of the AtlanticâÂÂCongo family. It includes all AtlanticâÂÂCongo except the families of the erstwhile Atlantic and Kordofanian branches and possibly Senufo.
In the infobox at the right, the languages which appear to be the most divergent (including the dubious Senufo) are placed at the top, whereas those closer to the core (the similar "BenueâÂÂKwa" branches of Kwa, VoltaâÂÂNiger and BenueâÂÂCongo) are near the bottom. If the Kwa or Savannas branches prove to be invalid, the tree will be even more crowded.
Comparative linguistic research by John M. Stewart in the sixties and seventies helped establish the genetic unity of VoltaâÂÂCongo and shed light on its internal structure, but the results remain tentative. Williamson and Blench (2000) note that in many cases it is difficult to draw clear lines between the branches of VoltaâÂÂCongo and suggest that this might indicate the diversification of a dialect continuum rather than a clear separation of families. This had been suggested before by Bennet (1983 as cited in Williamson and Blench 2000:17) in the case of the Gur and AdamawaâÂÂUbangi languages, which apart from Ubangian are now linked together as Savannas. Other branches are Kwa and BenueâÂÂCongo, which includes the well-known and particularly numerous Bantu group. The relationship of Kwa to BenueâÂÂCongo (named BenueâÂÂKwa), and the eastern and western branches of BenueâÂÂCongo to each other, remain obscure.
The vowel systems of VoltaâÂÂCongo languages have been the subject of much historical comparative linguistic debate. Casali (1995) defends the hypothesis that Proto-VoltaâÂÂCongo had a nine- or ten-vowel system employing vowel harmony and that this set has been reduced to a seven vowel-system in many VoltaâÂÂCongo languages. The GhanaâÂÂTogo Mountain languages are examples of languages where nine- or ten-vowel systems are still found.