Historical linguistic theory and practice contains a great number of different "layers" which have been accepted in the course of time and have acquired a permanency of their own. These range from neogrammarian conceptualizations of sound change and analogy to present-day ideas on rule change and language mixture. To get a full grasp of the principles of historical linguistics it is therefore necessary to understand the nature and justifications (or shortcomings) of each of these "layers", not just to look for a single "overarching" theory. The major purpose of the book is to provide in up-to-date form such an understanding of the principles of historical linguistics and the related fields of comparative linguistics and linguistic reconstruction. In addition, the book provides a very broad exemplification of the principles of historical linguistics.