Sibilants are consonants made with a hissing sound, in Classisal Adûnaic [s] and [z]. These were either survivals of the same Primitive Adûnaic sounds, or developed from voiceless and voiced Primitive Adûnaic palatals, which Tolkien labeled the c-series (SD/418). Tolkien did not define Adûnaic sibilants as its own group, but the discussion here does so to simplify discussion of the Primitive Adûnaic c-series.