Tibeto-Burman languages
In Matisoff's reconstruction Proto-Tibeto-Burman vowels can be split into primary and secondary sets. Modern-day Tibeto-Burman languages have anywhere from five vowels (Written Tibetan and Jingpho) to dozens of monophthongs and diphthongs (Loloish and Qiangic languages) (Matisoff 2003:157). Matisoff (2003) also notes that languages which have greatly simplified or eliminated final consonants tend to have more vowels. The open front unrounded vowel *a is by far the most common and stable vowel in Tibeto-Burman languages.