What is the relationship between phonetics and phonology? Are phonological features innate and universal, and do they have fixed phonetic correlates? These questions have received renewed prominence in theoretical debates, and this book explores them from a modular, substance-free perspective. This in-depth analysis of Breton serves not only to introduce important data from this endangered language into the theoretical landscape but also to demonstrate the viability of a modular phonological framework. The book introduces a minimalist system of phonological representations built up on a language-specific basis, and integrates it with a fully-fledged computational framework, showcasing the numerous empirical and conceptual advantages of a substance-free view of phonology.Presenting the first comprehensive analysis of the sound patterns of a Breton variety treated in a substance-free phonological framework, this book will enhance the understanding of Celtic phonology and offers a valuable reference for postgraduate students, academics and researchers working in phonological theory and Celtic studies.