In Sino-Tibetan (1972) P. K. Benedict has offered two different accounts of the significance, for Tibeto-Burman (TB) and Sino-Tibetan (ST) reconstruction, of the three types of ‘vocalic alternation’ in the Written Tibetan (WT) verb, a/a/o, e/a/o and o/a/o. His original account (c. 1942–3) reads: ‘Tibetan, however, shows a puzzling type of vocalic alternation in its verbs, in which stems in a regularly take o in the imperative and often either o or e in the present… The e of the present stem is possibly to be interpreted as an effect of the prefixed element’- <*a- [ə-]. Similarly, the o of the imperative stem has perhaps been conditioned by an archaic imperative suffixc -o…' (pp. 126–7). His revised version (1972) reads: ‘The Chinese vowels cannot be explained without setting up a 7-vowel system for ST … and Tibetan verb forms reflect this early system as follows:
ST/TB *a = T aa (no alternation, except in the imperative)
ST/TB *â = T ao
ST/TB *ə = T ae
We can now, by way of illustration, reconstruct TB *g-sât (T gsod-pa, Pf.bsad) …; also TB*səm “breath, voice, spirit”: T sem(s)-pa. Pf. semsbsams “think” …' (p. 126, n. 344).