Abstract The present article sets two goals: (i) to investigate the inner-Tocharian evolution of the Proto-Indo-European vowel system in light of the general principles of vowel shifts; (ii) to examine not only how the Proto-Indo-European vowel system led to the Proto-Tocharian ( PT ) vowel system, but also how the latter system further evolved in the attested Tocharian languages. As for (i), I argue the Pre-Proto-Tocharian vowel shifts to have been triggered by two fundamental changes, namely, the fronting of Pre- PT */ɔ/ to */ɛ/ and the assimilation of Pre- PT */iu̯/ to */uu̯/. As for (ii), I argue that, in Tocharian B, the central vowels /a, ǝ/ (< PT */a, ǝ/) developed the higher allophones ǝ, ɨ when unstressed. In Tocharian A, instead, the PT vowel system underwent a chain shift, whose trigger was the monophthongization of the PT diphthongs */ɛi̯, ai̯, ? ɔi̯/ and */ɛu̯, au̯/.
Giulio Imberciadori (Mon,) studied this question.