Vol. 36 Issue 1 Reviews  Reviews > Publications >  
Dmitri Tymoczko: A Geometry of Music: Harmony and Counterpoint in the Extended Common Practice  
Hardcover, 2011, ISBN 9780195336672, 450 pages with illustrations, index, bibliographic references, and online examples, US $39.95; available from Oxford University Press, 198 Madison Avenue, New York, New York 10016, USA; telephone (+1) 2127266000; http://www.oup.com; online examples and supporting material: http://www.oup.com/us/companion.websites/9780195336672/examples/?view=usa/. Reviewed by Michael Gogins I believe A Geometry of Music (hereinafter AGM) is a groundbreaking book in music theory. According to Dmitri Tymoczko, “While my stated audience consists of composers and music theorists, I have tried to write in a way that is accessible to students and dedicated amateurs.” If my own experience is any guide, AGM will be especially useful to composers who, like myself, use computers in composition. Indeed, this review is written mainly from the viewpoint of an algorithmic composer. But, as much as my informal education in music theory permits, I will also attempt to give the book some historical and theoretical context. AGM synthesizes about 15 years of work by the author (and some other theorists) towards developing a geometric understanding of many phenomena in voice leading, chord structure, chord progression, scale theory, and modulation. The starting point is to define each chord as a single point in a continuous Euclidean “chord space” with as many dimensions of pitch as the chord has voices. This simple idea turns out to be tremendously fruitful. Tymoczko convincingly argues that all commonly used measures of voiceleading distance agree with the length of the distance from one chord point to another chord point in chord space. AGM proposes that music which is tonal in the broadest meaning of the term, from the beginnings of Western polyphony until the extended tonality of today, and across many different classical and popular styles, features conjunct melodic motion, acoustic consonance, harmonic consistency, limited macroharmony (the notion of scale, more or less), and centricity (having a tonal center in the standard sense). I suspect that for academic theorists, the main interest of AGM will be its use of the geometric definition of voiceleading distance to develop a deeper understanding of these phenomena. According to Tymoczko’s theory, a chord is a point, and voiceleading is movement from one point to another point. Of course, music theorists use many different levels of abstraction in thinking about chords and scales. Theorists almost always abstract from the order of the voices, they usually ignore the particular octave of a pitch, and sometimes even ignore the particular inversion of a chord. And they generally ignore voice doublings. AGM shows that each of these levels of abstraction exactly corresponds to what mathematicians call an “equivalence class” in chord space. For example, the standard definition of a “pitchclass set” corresponds to combining the equivalence classes for octave (“O”), order of voices (“P” for permutation), and number of voices (“C” for cardinality): OPC. Other equivalence classes are “T” (for translational equivalence, i.e., OPTC equivalent chords are the same chord type) and “I” (for inversional equivalence). Tymoczko shows that the various chord spaces derived from each equivalence class, or combination of equivalence classes, inherit a “metric” or measure of distance from their parent, in purely Euclidean chord space. This is the signal fact that enables the concept of voiceleading distance to be used in more or less the same way with respect to pitchclass sets (OPC equivalence), chord types (OPTC equivalence), and setclasses (OPTIC equivalence). AGM further demonstrates that the concept of voiceleading, and of minimal voiceleading operations, applies not only to chords and chord progressions but, just as well, to scales and modulations. Indeed, Tymoczko argues, the use of the same measure of voiceleading distance for both chord progression and modulation provides a geometric foundation for understanding something many musicians feel: that there is a kind of selfsimilarity between chord progressions in the small scale and modulations in the mediumtolarge scale. AGM goes on to discuss atonal music, chromatic harmony, the structure of typical and normative chord progressions, modern and contemporary uses of variant scales, and many other matters using the underlying geometry of chord space with its measure of voiceleading distance as the unifying principle. Additionally, AGM draws on examples beginning with the twovoice counterpoint of the Middle Ages and ending with the extended tonality of Dmitri Shostakovitch, Bill Evans, and Steve Reich. Audio clips of these examples are accessible to the public on the Web site accompanying the book listed above. These online recordings and score excerpts greatly increase the usefulness of the book. Readers with keyboard skills would benefit from playing through the examples. Readers without such skills would benefit from careful listening to the online audio clips while following the score excerpts. This book is a further step along the path that began when Pythagoras identified musical intervals with numerical ratios, has passed through stages of increasing mathematical sophistication with the Tonnetz of Leonhard Euler and Arthur von Oettingen and the voiceleading operations of Hugo Riemann, and is now leading to something of an efflorescence of atonal set theory, group theory, neoRiemannian theory, and geometry. Where set theorists such as Milton Babbitt (“Some Aspects of TwelveTone Composition”, The Score and IMA Magazine 12, 1955), Allen Forte (The Structure of Atonal Music, Yale University Press, 1973) and their school focus on combinatorics and group theory applied to a discrete representation of pitch, Tymoczko (like some contemporary transformational or neoRiemannian theorists; see below) takes a mathematical step back to use a wider range of abstract algebra and geometry applied to a continuous representation of pitch. Furthermore, AGM demonstrates that some of the central phenomena of tonality, such as the “faithfulness” of the circle of fifths in reflecting relationships that obtain in higherdimensional chord spaces, cannot be adequately understood without a continuous representation of pitch. I believe this alone suffices to establish the theoretical importance of the book. One of the things AGM is trying to do is to provide a simpler view of music theory, based on principles rather than lists of rules and exceptions. The book certainly does this for me. Matters I formerly found more or less opaque, such as how those German and French sixths really work, or why some key changes are more common than others, or how tritone substitutions fit in, are now far clearer. So for me, the unification Tymoczko is attempting has in some part been achieved. Another thing that distinguishes AGM from much writing on music theory I have seen is its ambition towards, and occasional achievement of, empiricism: comparison with data, in this case analyses of all the Mozart piano sonatas and some 70 Bach chorales. I do not think music theory must be empirical to be interesting or valid, but I do think more experimentation in music theory can bring about innovation. In his conclusion, Tymoczko writes “my initial goal in writing this book was to explore basic theoretical and compositional issues,” but as his writing progressed, he found that the “five features could also provide a helpful framework for understanding the development of Western music.” Despite the gaps and spotty context, I definitely agree. For composers, I think what is most salient about AGM is that the “moves” we commonly use to get from one chord to another show up as short, typical movements in chord space with clear motivations that take one chord, usually not far from the central axis of augmented chords, to another. The same is true of the “moves” we use to get from one key to another, or to mutate between one scale and another. It is very nice, as a composer, to find a new perspective from which to view the landscape of music. But, as an algorithmic composer, it is stunning to be presented with a toolkit that brings efficient voiceleading, Riemannian and neoRiemannian transformations, scale theory, and more, so transparently into the engine room of algorithmic composition. Most importantly, the geometric approach makes it easy to turn analytical operations into generative ones because the mathematics is simple. Once the notions of equivalence class and quotient space have been assimilated, the rest is no more difficult than highschool algebra and basic group theory. And, operations implemented geometrically can be highly efficient. For example, automatically finding a counterpoint for a cantus firmus using the rules of Gradus ad Parnassum requires pages of code and exponential time, while doing it by looking up the nearest (by Euclidean distance) voicings of a series of twovoice consonant chords in a dictionary of such voicings requires a few dozen lines of code and logarithmic time.

