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', ...