The Batak languages can be divided into two main branches, Northern Batak and Southern Batak. Simalungun was long considered an intermediary, but in current classifications it is recognized as part of the Southern branch.[4][5] Within Northern Batak, a study noted 76% cognate words between Karo and Alas, 81% with Pakpak, 80% with Simalungun, and 30% with Malay (Indonesian).[6] Karo and Toba Batak are mutually unintelligible.
Mandailing, Toba and Angkola are related to each other and mutually intelligible. Karo languages
are mutually intelligible with other Northern Batak languages named Alas – Kluet language's in the southern part of Aceh, and are also partially mutually intelligible with Pakpak and Singkil. Some Pakpak (Dairi) dialect also partially mutually intelligible with Toba languages. Simalungun languages are sometimes partially mutually intelligible with both Northern and Southern Batak, but more comprehensible with other Southern Batak languages (Toba-Angkola-Mandailing). The geographical influences on the Batak languages can be seen in the map in the infobox; Lake Toba separates the Karo (Northern Batak) from direct contact with the Toba (Southern Batak).
The Batak languages can be shown to descend from a hypothetical common ancestor, Proto-Batak (which in turn originates from Proto-Austronesian). The sound system of Proto-Batak was reconstructed by Adelaar (1981).[5]
Proto-Batak final voiced stops *b, *d, and *g are retained only in Simalungun. In Toba, Angkola and Mandailing, they are unvoiced, while in the Northern Batak languages, they changed to homorganic nasals (/m/, /n/, /ŋ/):
^Adelaar, Alexander K. (2010). "Language documentation in the west Austronesian world and Vanuatu: An overview". In Florey, Margaret (ed.). Endangered languages of Austronesia. Oxford: Oxford Univ. Press. p. 24. ISBN978-0-19-954454-7.
^Comparative Austronesian dictionary Vol. 1. by Darrell T. Tryon, Shigeru Tsuchida et al. p421 et seq
^ abcAdelaar, K. A. (1981). "Reconstruction of Proto-Batak Phonology". In Robert A. Blust (ed.), Historical Linguistics in Indonesia: Part I, 1–20. Jakarta: Universitas Katolik Indonesia Atma Jaya.
^The Austronesian languages of Asia and Madagascar. K. Alexander Adelaar, Nikolaus Himmelmann, p. 535