Tonal music, from a historical perspective, is far from homogenous; yet an enduring feature is a background "diatonic" system of exactly seven notes orderable cyclically by fifth. What is the source of the durability of the diatonic system, the octave of which is representable in terms of two particular integers, namely 12 and 7? And how is this durability consistent with the equally remarkable variety of musical styles -- or languages -- that the history of Western tonal music has taught us exist? This book is an attempt to answer these questions. Using mathematical tools to describe and...