There exist occasional cases of alternations between words with preradical d- and r- in Old/Written Tibetan (OT/WT), for instance dba vs rba ‘wave’, dgu ‘nine’ vs. rgu ‘many’. I am uncertain of the nature (phonological ? dialectal ?) of these alternations. At the same time, rb-type onsets are rare in Written Tibetan and rp- onsets are entirely absent. In a blog dated 19/11/2015 (https://panchr.hypotheses.org/527) Guillaume Jacques proposed that a metathesis has affected pre-Tibetan *rp-, changing it to WT phr-; this is supported by a comparison between WT phrag-pa and Japhug tɯ-rpaʁ, both ‘shoulder’, both forms being derivable from an earlier *rpak. Combining Jacques’ hypothesis with r-/d- preradical doubleting stimulates us to look for db-/br and dp-/phr- doublets. Here is an apparent instance of a db-/br- doublet: dbur ‘to grind, pulverize; flour’ vs. brul ‘very small broken pieces’. These two forms could go back to a dbur vs. rbur doublet, assuming the evolution from rbur to brul involves a dissimilatory change of final -r to -l.
Cite this post as: Laurent Sagart, "A possible case of dissimilation of -r to -l in Written Tibetan," in Sino-Tibetan-Austronesian, 12/06/2017, https://stan.hypotheses.org/11.