In 816 CE, during the reign of Sadnalegs, literary Tibetan underwent a thorough reform aimed at standardizing the language and vocabulary of the translations being made from Indian texts, and this resulted in what we now call Classical Tibetan.
Old Tibetan is characterised by many features that are lost in Classical Tibetan, including my- rather than m- before the vowels -i- and -e-, the cluster sts- which simplifies to s- in Classical Tibetan, and a reverse form of the "i" vowel letter (gi-gu). Aspiration was not phonemic and many words were written indiscriminately with consonants from the aspirated or unaspirated series. Most consonants could be palatalized, and the palatal series from the Tibetan script represents palatalized coronals. The sound conventionally transcribed with the letter འ (Wylie: 'a) was a voiced velar fricative, while the voiceless rhotic and lateral are written with digraphs ཧྲ <hr> and ལྷ <lh>. The following table is based on Hill's analysis of Old Tibetan:
In Old Tibetan, syllables can be quite complex with up to three consonants in the onset, two glides, and two coda consonants. This structure can be represented as (C1C2)C3(G1G2)V(C4C5), with all positions except C3 and V optional. This allows for complicated syllables like བསྒྲིགས bsgrigs "arranged" and འདྲྭ 'drwa "web", for which the pronunciations [βzgriks] and [ɣdrʷa] can be reconstructed.
A voicing contrast only exists in slot C3 and spreads to C1 and C2 so སྒོ sgo "door" would be realized as [zgo] while སྐུ sku "body" would be [sku]. Final consonants are always voiceless e.g. འཛིནད་ 'dzind [ɣd͡zint] and གཟུགས་ gzugs [gzuks]. The phoneme /b/ in C1 was likely realized as [ɸ] (or [β] when C3 is voiced) e.g. བསྒྲེ bsgre [βzgre] and བརྩིས brtsis [ɸrtsis]. The features of palatalization /i̯/ [Cʲ] and labialization /w/ [Cʷ] can be considered separate phonemes, realized as glides in G1 and G2 respectively. Only certain consonants are permitted in some syllable slots, as summarized below:
§ In C2 position, /d/ and /g/ are in complementary distribution: /g/ appears before /t/, /ts/, /d/, /n/, /s/, /z/, /l/, and /l̥/ in C3, while /d/ appears before /k/, /g/, /ŋ/, /p/, /b/, and /m/ in C3.
Palatalization /Cʲ/ was phonemically distinct from the onset cluster /Cy/. This produces a contrast between གཡ <g.y> /gj/ and གྱ <gy> /gʲ/, demonstrated by the minimal pair གཡང་ g.yaṅ "sheep" and གྱང་ gyaṅ "also, and". The sounds written with the palatal letters ཅ c, ཇ j, ཉ ny, ཞ zh, and ཤ sh were palatalized counterparts of the phonemic sounds ཙ ts, ཛ dz, ན n, ཟ z, and ས s.
However, whereas the locative, allative, and terminative gradually fell together in Classical Tibetan (and are referred to the indigenous grammatical tradition as the la don bdun), in Old Tibetan these three cases are clearly distinguished. Traditional Tibetan grammarians do not distinguish case markers in this manner, but rather distribute these case morphemes (excluding -dang and -bas) into the eight cases of Sanskrit.
Old Tibetan has three first person singular pronouns ང་ ṅa, བདག་ bdag, and ཁོ་བོ་ kho-bo, and three first-person plural pronouns ངེད་ nged, བདག་ཅག་ bdag-cag, and འོ་སྐོལ་ 'o-skol. The second person pronouns include two singulars ཁྱོད་ khyod and ཁྱོ(ན)་འདའ་ khyo(n)-'da' and a plural ཁྱེད་ khyed.