Ao languages

The Ao or Central Naga languages are a small family of Sino-Tibetan languages spoken by various Naga peoples of Nagaland in northeast India. Conventionally classified as "Naga", they are not clearly related to other Naga languages, and are conservatively classified as an independent branch of Sino-Tibetan, pending further research. There are around 607,000 speakers of the languages in total.

Coupe (2012) considers the Angami–Pochuri languages to be most closely related to Ao as part of a wider Angami–Ao group.

Languages
The following languages are widely accepted as Central Naga languages: There are also various undescribed Ao varieties including Yacham and Tengsa, which may turn out to be separate languages (see Mongsen Ao).
 * Ao language
 * Chungli Ao
 * Mongsen Ao
 * Sangtam ('Thukumi')
 * Yimkhiungrü ('Yachumi')
 * Lotha (Lhota)

The following "Naga" languages spoken in and around Leshi Township, Myanmar are classified as Ao languages ("Ao-Yimkhiungrü") by Saul (2005).
 * Koki
 * Makury
 * Long Phuri
 * Para

Hsiu (2021) places Makury, Long Phuri, and Para into a Greater Central Naga branch, but excludes Koki (Kokak).


 * Greater Central Naga
 * Makury
 * Long Phuri
 * Para (Jejara)
 * Central Naga
 * Lotha
 * Sangtam
 * Yimchungrü
 * Ao

Bruhn (2014:370) also surmises that Makury may be an Ao language.

Bruhn (2014) uses the term Central Naga to refer to all of the languages above, and uses the Ao to refer to only two languages, namely Chungli Ao and Mongsen Ao. The internal structure of Bruhn's Central Naga group is as follows.
 * Central Naga
 * Lotha
 * Sangtam
 * Yimkhiungrü
 * Ao
 * Chungli Ao
 * Mongsen Ao

Coupe (2023) suggests that Wui, a recently described divergent language of eastern Nagaland, is likely a divergent Aoic (i.e., Central Naga) language.

Reconstruction
Proto-Central Naga (Proto-Ao) has been reconstructed by Bruhn (2014).

Bruhn (2014:363) identifies the following four sound changes from Proto-Tibeto-Burman (PTB) to Proto-Central Naga (PCN) as sound changes that are characteristic of the Central Naga branch.
 * 1) PTB *-a(ː)w, *-əw, *-ow, *-u > PCN *-u(ʔ) ‘back diphthong merger’
 * 2) PTB *-r > PCN *-n ‘*r-coda nasalization’
 * 3) PTB *-s > PCN *-t ‘*s-coda occlusivization’
 * 4) PTB *-i(ː)l, *‑al, *‑uːl > PCN *‑ə(ʔ) ‘*l-rime erosion’