Based on the divergent functions attested in Purik for all four stems of the maximally complex transitive Written Tibetan (WT) verb paradigms, we are able to reconstruct a Proto-Tibetan (PT)verb system in which labial-prefixed voiceless onsets triggered a focus on the initial phase of an event (i.e., its instigation), nasal-prefixed voiced  onsets  on  its final phase  (i.e., its  result), and unprefixed  and  eventually aspirated voiceless onsets on the event as such (or the middle phase of the event). The reconstruction of this threefold phasal distinction for PT allows us to recognize the original functions of a number of other features of Tibetan verbal morphology, to wit, the “stative” -s suffix, the nominalizing -d suffix, the causative s- prefix and its “result-causative” form z-, and the “deictic” -o- replacing the stem vowel -a-. Furthermore, the most plausible and economic account for how all these features evolved in different varieties of Tibetan involves the assumption that subordinator-less concatenations were common in PT when two verbs described different facets of one and the same event.