Research on ferroelastic crystals and related materials has obtained some aspects of a tradition but has not yet mellowed. Vigorous discussion on serious questions such as the underlying physical principles which generate elastic instabilities have just begun whilst debates concerning nomenclature still linger on. Application of the physical concepts in Earth Sciences, Material Sciences and Solid State Chemistry are new and growing rapidly. To write a monograph at this stage of development must be premature but if this book is perceived by the reader as a stimulus rather than the outline of the final concepts, it has fulfilled my hopes.

A few words of apology are called for if the reader is not to criticise the book more sharply than it doubtless deserves.

There are reviews on ferroelasticity, such as that by Wadhawan, and many an article dedicated to the illumination of symmetry concepts as started so early by Aizu, the Czech school of Janovec, Dvorak and Petzelt, the innovative French Toledano brothers, and the recent contributions by Hatch and Stokes. All this work deserves the highest credit. None of them, so far as I know, has quite the same purpose as this book, however. Pure symmetry considerations are aesthetically satisfying and are very useful, in particular for classification purposes, when all the physical facts are known. The concept that all ferroelastic phase transitions always follow the simple track of direct group-theoretical predictions of the most simplistic manner, however, died some years ago.