Plains Apache is most closely related to other Southern Athabaskan languages like Navajo, Chiricahua Apache, Mescalero Apache, Lipan Apache, Western Apache, and Jicarilla Apache. Plains Apache is the most divergent member of the subfamily. The language is extremely endangered with perhaps only one or two native speaking elders. Alfred Chalepah, Jr., who might have been the last native speaker, died in 2008.[citation needed]
Bittle, William E. (n.d.). Plains Apache field notes. (Unpublished manuscript).
Bittle, William E. (1956). The position of Kiowa-Apache in the Apachean group. (Doctoral dissertation, University of California, Los Angeles).
Bittle, William E. (1963). Kiowa-Apache. In H. Hoijer (Ed.), Studies in Athabaskan languages (pp. 76–101). University of California publications in linguistics (No. 29). Berkeley: University of California Press.