Se llama temperamento justo a la afinación de los instrumentos musicales que sigue la norma de adoptar en lo posible los intervalos de la serie armónica, en particular la tercera mayor. Así como en el sistema de Pitágoras tenemos una tercera mayor de razón 81/64, formada por cuatro quintas perfectas o puras de razón 3/2, y que recibe el nombre de ditono pitagórico, en el sistema justo se reduce este intervalo hasta igualarse a la tercera mayor "justa" o "pura" que existe entre los armónicos 4 y 5 de la serie armónica. Cuando se calcula la diferencia entre la tercera mayor pura y el ditono, se obtiene un intervalo de razón 81/80 llamado coma sintónica. Para conseguir la reducción de las terceras mayores, partiendo del círculo de quintas pitagórico se toma una quinta de cada cuatro (pues la tercera mayor consta de cuatro quintas) y se reduce precisamente en una coma sintónica. Las quintas reducidas del sistema justo tienen una proporción de 3/2 : 81/80 = 40/27.

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In music, just intonation (sometimes abbreviated as JI) or pure intonation is any musical tuning in which the frequencies of notes are related by ratios of small whole numbers. Any interval tuned in this way is called a pure or just interval. Pure intervals are important in music because they naturally tend to be perceived by humans as "consonant": pleasing or satisfying. Intervals not satisfying this criterion, conversely, tend to be perceived as unpleasant or as creating dissatisfaction or tension. The two notes in any just interval are members of the same harmonic series. Frequency ratios involving large integers such as 1024:729 are not generally said to be justly tuned. The Indian classical music system uses just intonation tuning as codified in the Natya Shastra. "Just intonation is the tuning system of the later ancient Greek modes as codified by Ptolemy; it was the aesthetic ideal of the Renaissance theorists; and it is the tuning practice of a great many musical cultures worldwide, both ancient and modern." Just intonation can be contrasted and compared with equal temperament, which dominates Western instruments of fixed pitch (e.g., piano or organ) and default MIDI tuning on electronic keyboards. In equal temperament, all intervals are defined as multiples of the same basic interval, or more precisely, the intervals are ratios which are integer powers of the smallest step ratio, so two notes separated by the same number of steps always have exactly the same frequency ratio. However, except for doubling of frequencies (one or more octaves), no other intervals are exact ratios of small integers. Each just interval differs a different amount from its analogous, equally tempered interval. Justly tuned intervals can be written as either ratios, with a colon (for example, 3:2), or as fractions, with a solidus (3 ⁄ 2). For example, two tones, one at 300 hertz (cycles per second), and the other at 200 hertz are both multiples of 100 Hz and as such members of the harmonic series built on 100 Hz. Thus 3/2, known as a perfect fifth, may be defined as the musical interval (the ratio) between the second and third harmonics of any fundamental pitch.

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