Abstract This chapter focuses on Les sons du français, Passy’s seminal work, which appeared in twelve editions. It traces the evolution of his scientific thinking through the revisions and additions made across these editions. Beyond a detailed overview of the segmental system, the chapter offers an in-depth analysis of Passy’s original views on stress—particularly initial stress—and the prosodic structure of verse, the palatal nasal consonant, schwa, and liaison behaviour. The chapter demonstrates that both schwa and liaison patterns have remained remarkably stable since Passy’s time. It also takes into account variation across different speech registers and highlights Passy’s progressive approach to variation and linguistic norms in language teaching.