What are the main features reconstructed for Proto-Tibeto-Burman?

Reconstructing Proto-Tibeto-Burman (PTB) involves identifying the phonological and structural features of the ancestral language. Scholars have reached a consensus on many of these features, though some aspects remain debated.1

Syllable Structure

PTB is reconstructed as a monosyllabic language, though it featured complex syllable structures that could be schematized as:
(P1) (P2) C_i (G) V^[T](:) (C_f) (s)

• Initial Consonants (C_i): These could be preceded by up to two consonantal prefixes (P1, P2) and followed by a glide (G).
• Vowels (V): Vowel length was contrastive, though this was primarily limited to syllables closed by a stop, nasal, liquid, or semivowel.2
• Final Consonants (C_f): Nine final consonants are reconstructed: -p, -t, -k, -m, -n, -ŋ, -r, -l, and -s.3
• Tone: The status of contrastive tone at the PTB stage is uncertain. Many scholars believe tone developed independently in various branches rather than being a feature of the proto-language.2

Morphological Features

• Affixation: PTB utilized prefixes and suffixes, which contributed to the formation of initial and final consonant clusters.4
• Grammaticalization: Abstract grammatical relationships were often expressed through the specialization of full nouns and verbs, which eventually underwent phonological reduction.5
• Agreement Systems: While some modern languages exhibit complex agreement, it remains unclear if these systems existed in PTB or arose independently.5