Historical linguistic theory and practice consist of a large number of chronological 'layers' that have been accepted in the course of time and have acquired a permanence of their own. These range from neogrammarian conceptualizations of sound change, analogy, and borrowing, to prosodic, lexical, morphological, and syntactic change, and to present-day views on rule change and the effects of language contact. To get a full grasp of the principles of historical linguistics it is therefore necessary to understand the nature of each of these 'layers'. This book is a major revision and reorganization of the earlier editions and adds entirely new chapters on morphological change and lexical change, as well as a detailed discussion of linguistic palaeontology and ideological responses to the findings of historical linguistics to this landmark publication.