This book has been a long time in the making. My work on features and segmental structure started in the early 1980s when I began exploring the new wave of autosegmental and metrical theories, applying the former to vowel harmony cases (leading to another book that was long in the making: van der Hulst (2018)) and the latter to my analysis of syllable structure and stress in Dutch (van der Hulst (1984)). Around that same time, learning about Dependency Phonology from my colleague Colin Ewen, I started considering the use of single-valued features and dependency. In 1990, I finished a manuscript that was entitled ‘The book of segments’, which I distributed on a small scale. This manuscript (essentially a forerunner of the present book), which contained an ambitiously ‘complete’ account of segmental representation in terms of unary features and dependency, has been the backbone of a lot of my work in this area since then. For each paper or talk on this subject, I would update the theory, which, in my view at least, each time made it better, giving it wider empirical scope and greater theoretical simplicity and elegance. In this endeavour, I collaborated with various colleagues such as Colin Ewen, Marcel den Dikken, Helga Humbert, Maarten Mous and Norval Smith. The model underwent many changes, slowly moving to an approach that uses a minimal number of phonological primes. Along the way, it became clearer to me what I was trying to achieve with my attempt to develop a structure that would account for all phonological ‘features’ and their interrelationships. In the early days of phonology, the basic units that form the perceptual side of language were thought to be ‘speech sounds’, or more technically phonemes, meaningless mental units of sound that in linear sequences would form meaningful units like morphemes and words. Phonemes were taken to be the ‘atoms’ of language. A new development introduced units that are smaller than phonemes, called (distinctive) features, which stand for properties of speech sounds, or could be seen as building blocks of phonemes.