The split of IE *VRs and *VsR into VRR in Lesbian and Thessalian and VR in the other dialects cuts a striking isogloss through ancient Greece. This split may be related to other developments of Greek by applying Jakobson's feature system to four Greek sound changes, all of them in essence well known and widely accepted. It is then shown that this suffices to solve, with no modification of the rules, the problem of the reflexes of IE *ws and *sy in Greek, and accounts for such hitherto ill-explained developments as *awsōs > Doric āwṓs, Lesbian aúōs ‘dawn’; *awsēr > Ionic āḗr, Lesbian aúēr ‘lower atmosphere’; *dewsomai > Attic déomai, Lesbian and Thessalian deúomai ‘need’; *dhusyō > Ionic thūō, Aeolic thuíō ‘rage’; and gen. *-osyo > Attic and Ionic *-ōo > -ō, Aeolic -oio.