This short explanation of Just Intonation was originally written for the Just Intonation Network website, and also appeared, in slightly modified forms, in various JIN promotional materials. For a longer version, see the Introduction to the Just Intonation Primer.
JUST INTONATION is any system of tuning in which all of the intervals can be represented by ratios of whole numbers, with a strongly-implied preference for the smallest numbers compatible with a given musical purpose. Unfortunately this definition, while accurate, doesn't convey much to those who aren't already familiar with the art and science of tuning. The aesthetic experience of just intervals and chords, however, is unmistakable.
The simple-ratio intervals upon which Just Intonation is based are the fundamental constituents of melody and harmony. They are what the human auditory system recognizes as consonance, if it ever has the opportunity to hear them in a musical context. The significance of whole-number ratios has been recognized by musicians around the world for at least five thousand years.
Just Intonation is not a particular scale, nor is it tied to any particular musical style. It is, rather, a set of principles which can be used to create a virtually infinite variety of intervals, scales, and chords which are applicable to any style of tonal music (or even, if you wish, to atonal styles). Just Intonation is not, however, simply a tool for improving the consonance of existing musics; ultimately, it is a method for understanding and navigating through the boundless reaches of the pitch continuum—a method that transcends the musical practices of any particular culture.
Just Intonation has depth and breadth. Its fundamental principles are relatively simple but its ramifications are vast. At present, Just Intonation remains largely unexplored. A few pioneering composers and theorists have sketched in some of its most striking features, but the map still contains many blank spaces where the adventuresome composer many venture in hopes of discovering new musical treasures.
In light of its numerous virtues, why isn't Just Intonation currently in general use? Like so many of our peculiar customs, it is largely an accident of history. During the sixteenth, seventeenth, and eighteenth centuries, when Western harmonic music and keyboard instruments were coevolving, instrument technologies were inadequate to the task of developing affordable, playable instruments that could accommodate the intricacies of Just Intonation. As a result, various compromises or temperaments were attempted. Twelve-tone equal temperament was ultimately adopted because it provided the greatest facility for transposition and modulation with the smallest number of tones, and because it made all of the intervals of a given type equally out of tune, thus avoiding the contrast between in-tune and out-of-tune intervals that characterized some earlier temperaments.
Equal temperament was not adopted because it sounded better (it didn't then, and it still doesn't, despite 150 years of cultural conditioning) or because composers and theorists were unaware of Just Intonation. The adoption of twelve-tone equal temperament was strictly a matter of expediency. Equal temperament allowed eighteenth- and nineteenth-century composers to explore increasingly complex harmonies and abstruse modulations, but this benefit was short-lived. By the beginning of the twentieth century, all of the meaningful harmonic combinations in the equally tempered scale had been thoroughly explored and exploited, and many composers believed that consonance, tonality, and even pitch had been exhausted as organizing principles. What was really exhausted was merely the limited resources of the tempered scale. By substituting twelve equally spaced tones for a universe of subtle intervallic relationships, the composers and theorists of the eighteenth and nineteenth centuries effectively painted Western music into a corner from which it has not yet succeeded in extricating itself.
Fortunately, a few visionary composers, most notably Harry Partch and Lou Harrison, rediscovered the source of truly viable new musical resources. These farsighted musicians recognized that in the acoustically pure intervals of Just Intonation, and in the diverse traditions of World music were to be found sufficient material to fruitfully occupy generations of composers. Unfortunately, until recently composing and performing sophisticated music in Just Intonation presented such difficulties that only the most dedicated enthusiasts were likely to invest the required time and effort. However, due to the recent appearance of affordable electronic instruments with programmable tuning capabilities, it is now possible for almost any musician to explore Just Intonation without first making a major commitment. The technical barriers having been largely removed, the only thing lacking for a widespread growth in the use of Just Intonation is an increased awareness of intonational principles and their musical applications on the part of our more advenuresome musicians.