Chapter 4 looks in detail at selected issues of historical Central Chadic phonology, by first of all confronting hypotheses about regular ‘natural’ vs. sporadic ‘contact-induced’ sound changes. It then discusses at some length the vocalogenesis scenario by which a minimal vowel system becomes massively enriched via phonologisation of allophones and variants of reconstructed vowels and approximants. The chapter than discusses canonical root shapes and the potential functionality of root types, not least in the light of different sound changes affecting different root types. It then looks in some detail at the various markers that are used as root-augmental material, including a discussion of potential circumfix marking in the proto-language. It then introduces the hitherto neglected prosodies of ‘glottalisation’ and obstruent ‘prenasalisation’ and their historical origins in the proto-language. It concludes by discussing alternative diphthong and monophthong word endings.