Intonation
Intonation is the tuning of pitches in performance — the specific frequencies a musician produces when playing or singing. In fixed-pitch instruments (piano, organ, fretted guitar), intonation is largely determined by the instrument’s construction. In flexible-pitch instruments (voice, violin, trombone, unfretted strings) and in many wind instruments, the performer continuously adjusts intonation in real time.
This adjustment is not a matter of hitting a predetermined target more or less accurately. Intonation is an expressive and culturally shaped dimension of music-making. A blues singer bends a note between the minor and major third not because they are failing to hit one or the other but because the between is the note. The microtonal inflections of maqam performance, the pitch flexibility of Hindustani vocal music, and the “blue notes” of African American musical tradition are all intonational practices that carry musical meaning.
Equal temperament — the tuning system in which the octave is divided into twelve exactly equal semitones — has become the dominant fixed-pitch standard in Western music since the nineteenth century. It is a compromise: no interval except the octave is acoustically pure, but all keys are equally (im)pure, enabling unrestricted modulation. This compromise has been enormously productive for Western harmonic practice, but it has also trained Western ears to hear its particular set of pitch relationships as “in tune” and all others as deviations.