There are considered two main Batak language groups, Northern Batak and Southern Batak. Simalungun has been considered an intermediary, but more recent studied suggest that it is a part of the Southern Batak group. Within Northern Batak, a study noted 76% cognate words between Karo and Batak, 81% with Pakpak, and 80% with Simalungun, and 30% with Malay (Indonesian). Karo and Toba Batak are mutually unintelligible.
Mandailing and Angkola are closer related to each other than to Toba. The geographical influences on the Batak languages can be seen in the map to the right; Lake Toba separates the Karo from direct contact with the Toba.