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 80% cognate words between Karo and Alas, 81% with Pakpak, 76% with Simalungun & Toba, and 30% with Malay (Indonesian).

Some Pakpak (Dairi) dialect also partially mutually intelligible with Toba languages.

The Batak languages can be shown to descend from a hypothetical common ancestor, Proto-Batak (which in turn originates from Proto-Austronesian).

The Proto-Batak sounds underwent the following changes in the individual daughter languages:[5] Historically, the Batak languages were written using the Batak script, but the Latin script is now used for most writing.