Apart from prefixation and suffixation, only one general morphological process can be assigned to the parent TB speech, viz. alternation of root initial. This feature is present in a number of TB roots reconstructed above, viz. *bar∼*par ‘burn’, *be∼*pe ‘broken, break’, *bleŋ∼*pleŋ ‘straight, straighten’, *bliŋ∼*pliŋ ‘full, fill’, *brup∼*prup ‘overflow, gush’, *byar∼*pyar ‘affix, plait, sew’, *dup∼*dip, *tup∼*tip ‘beat’, *dyam∼*tyam ‘full’, *gwa-n∼*kwa-n ‘put on clothes’, *du-t∼*tu-t ‘join, tie, knot’, *bip∼*pip ‘conceal, bury’. In Tibetan, Kiranti, Bahing, Vayu, and Bodo-Garo the fundamental contrast is that between intransitives with sonant initials and transitives with surd initials, and this contrast surely is to be regarded as an inherited TB feature. No invariable relation existed between root initial and verbal function, as shown by transitive roots such as *dza ‘eat’ with sonant initial; we can state simply that certain roots show the alternation, while others do not.

The alternation of initial sonant and surd in Tibetan itself is obscured by extensive prefixation and the specialization of verb forms as ‘present’, ‘perfect’, ‘future’, or ‘imperative’, e.g. ʾbud-pa, Pf. and Imp. phud, Fut. dbud ‘put off, pull off’, also ʾphud-pa. As noted by Francke and Simon (in Jäschke, Tibetan Grammar), the main line of cleavage in Tibetan roots is that between presents and futures (sonant initial, intransitive or durative) and perfects and imperatives (surd initial, transitive or active).