This paper continues a series of treatments of sound changes in the Celtic languages that have not been satisfactorily or fully explained to date. Sound changes that occurred in proto-Brittonic and early Welsh are treated: (3) the shift of */j/ > /ð/ / ˈVr_V in proto-Brittonic; (4) the shift of the group *-/nthL/- > -/θL/- in Old Welsh; (5) the evolution of the group */lthr/ in Welsh.