Hill (2019: §33, §224) reconstructs a vowel *e in the Sino-Tibetan parent language (Proto-Trans-Himalayan in his terminology; I will be using the term PCT ‘Proto-Chino-Tibetan’, meaning the language one would reconstruct if only Old Chinese an Tibetan were available). He argues that this vowel is preserved unchanged in OC, and that its reflexes in WT are conditioned by the ending: one has WT e before labials, i before velars, and a before dentals (including –r and –l). Below are examples of the correspondence OC *e to Written Tibetan a in words without a coda (1), with labial codas (2-3), velar codas (5-6), PCT *-q (7) and PCT *-ʔ (8-9)
OC *e : WT a
1. 溪 *kʰˤe > khej > xī ‘valley with stream in it’ : WT rka ‘small furrow conveying water from a conduit to trees and plants’
2. 占 *tem > tsyem > zhān ‘prognosticate’ : WT gtam ‘talk, discourse, speech’
3. 攝 *kə.n̥ep > syep > shè ‘catch, gather up’ : WT rnyab ‘seize or snatch together’
4. 貞 *treŋ > trjeng > zhēn ‘straight; correct’ : WT draŋ-po ‘straight’
5. 脛 *m-kʰˤeŋ-s > hengH > jìng ‘leg, shank’, 牼 *m-kʰˤeŋ > heang > kēng ‘shank bone’ : WT rkang ‘marrow, leg bones, stalk’
6. 清 *tsʰeŋ > tshjeng > qīng ‘clear (adj.)’, 淨 *m-tseŋ-s > dzjengH > jìng ‘cleanse (v.t.)’ : WT g-tsang-ba ‘to be clean, pure’
7. 舐 *Cə.leʔ > zyeX > shì ‘lick’ : WT ldag pa ‘to lick’
8. 庳 *N-peʔ > bjieX > bì ‘low, short’, 鼙 *[b]ˤe > bej > pí ‘small hand-drum’ : WT p’ra-ba ‘thin, fine, minute; little, small’
9. 諀 *pʰeʔ > phjieX > pǐ ‘slander’ : WT p’ra-ma ‘slander’
Similarly, the correspondence OC *e : Tibetan e is not limited to words with labial endings. Below are examples in words without a coda (1), ending in some kind of back-of-the-mouth fricative (2-3) and, in apparent violation of Dempsey’s law, in a velar (4-5):
OC *e : WT e
1. 佳 *[k]ˤre > kea > jia ‘good’ : WT dge ‘happiness, welfare, happy, propitious’
2. 秝 *[r]ˤek > lek > lì ‘successively, sequence’ : WT re ‘one at a time’, etc.
3. 譬 *pʰek-s > phjieH > pì ‘example’ : WT dpe ‘pattern, model; example, illustrative parable’
4. 積 *[ts]ek-s > tsjeH > jī ‘to hoard’ : WT rtseg-pa, pf. (b)rtsegs ‘to lay one thing on or over another, to pile up, stack up, build up’
5. 易 *lek-s > yeH > yì ‘easy’ : WT legs-pa ‘good, happy; neat, elegant, beautiful …’
These two correspondences of OC *e are not in complementary distribution.
Hill, Nathan W. 2019. The Historical Phonology of Tibetan, Burmese and Chinese. Cambridge: Cambridge University Press.