The closest relatives of the Mongolic languages appear to be the extinct Khitan and Tuyuhun languages. Some linguists have grouped Mongolic with Turkic, Tungusic, and possibly Koreanic and Japonic as part of the controversial Altaic family, but this has been widely discredited.
The classification and nunbers of speakers above follow Janhunen (2006) except for Southern Mongolic, which follows Nugteren (2011). In another classificational approach, there is a tendency to call Central Mongolian a language consisting of Mongolian proper, Oirat and Buryat, while Ordos (and implicitly also Khamnigan) is seen as a variety of Mongolian proper. Within Mongolian proper, they then draw a distinction between Khalkha on the one hand and Southern Mongolian (containing everything else) on the other hand. A less common subdivision of Central Mongolic is to divide it into a Central dialect (Khalkha, Chakhar, Ordos), an Eastern dialect (Kharchin, Khorchin), a Western dialect (Oirat, Kalmyk), and a Northern dialect (consisting of two Buryat varieties). The broader delimitation of Mongolian may be based on mutual intelligibility, but an analysis based on a tree diagram such as the one above faces other problems because of the close contacts between, for example, Buryat and Khalkha Mongols during history, thus creating or preserving a dialect continuum. Another problem lies in the sheer comparability of terminology l,as Western linguists use language and dialect, while Mongolian linguists use the Grimmian trichotomy language (kele), dialect (nutuγ-un ayalγu) and Mundart (aman ayalγu).
Proto-Mongolic, the ancestor language of the modern Mongolic languages, is very close to Middle Mongol, the language spoken at the time of Genghis Khan and the Mongol Empire. Most features of modern Mongolic languages can thus be reconstructed from Middle Mongol. An exception would be the voice suffix like -caga- 'do together', which can be reconstructed from the modern languages but is not attested in Middle Mongol.
One can speculate that the languages of Donghu, Wuhuan, and Xianbei might be related to Proto-Mongolic. For Tabghach, the language of the founders of the Northern Wei dynasty for which the surviving evidence is very sparse, and Khitan, for which evidence exists that is written in the two Khitan scripts which have as yet not been fully deciphered, a direct affiliation to Mongolic can now be taken to be most likely or even demonstrated.