T prefixed b- is characteristically found with the ‘perfect’ root of verbs, as in gsod-pa, Pf. bsad ‘kill’, yet occurs also with the ‘present’ root, as in ʾbri-ba ‘draw write’ < *riy. Wolfenden (Outlines, pp. 33 ff.) suggests that this prefix represents an ‘acting subject’. Bodo-Garo has a 3rd person pronominal element b- occurring independently (Bodo bi, Dimasa bo) and as a prefix, e.g. Dimasa bugur ‘skin’, as contrasted with sao-gur ‘human skin’, mi-gur ‘animal skin, hide’ (cf. n. 301). Confusion between prefixed *b- and *m- (a pronominal element) is widespread in Tibeto-Burman, e.g. Kachin and Meithei have m∂-, Nung has ph∂- (rarely b∂-), and many Kuki-Naga languages have either p- (Lakher, Northern Khami) or m- (Rangkhol, Southern Khami) for both prefixes. Mikir and Ao and Sema Naga, however, regularly maintain the distinction between *b- (Mikir ph-) and *m- (Mikir iŋ-), thus permitting the exact reconstruction of Kuki-Naga roots such as the following:

*b-la ‘cotton’: Mikir phelo, Lakher p∂la, N. Khami ph∂lo, S. Khami m∂hla, L la. Burmese has shifted *b- to *m- before *r- or *l- in three of the roots cited below (cf. mrup ‘submerged’ < TB *brup), yet has simply lè ‘4’ for TB *b-liy (but Maru byit < *bliy).