Just intonation
Tuning by pure intervals, for example fifth 3:2 and major third 5:4.
What it means
Just intonation is a tuning system in which the intervals are tuned according to simple numerical ratios that correspond to the harmonic series: the octave in the ratio 2:1, the fifth 3:2, the fourth 4:3, the major third 5:4 and the minor third 6:5. Pure intervals sound perfectly calm and without beats; the notes blend.
Musical use
In just intonation not every interval can be kept pure in every key: pure fifths and pure thirds do not fit together, because twelve pure fifths do not meet seven octaves. With a fixed just tuning only a few keys can therefore be played well, and modulations are limited. Choirs, string quartets and barbershop singers unconsciously adjust the notes in the chord to just intonation. Keyboard instruments use temperaments.
Just intonation should be distinguished from equal temperament, in which all semitones are the same size and all intervals except the octave deviate slightly, and from Pythagorean tuning, which uses only pure fifths.
Performance practice
When you play or sing in an ensemble without a keyboard instrument, listen for the beats between the notes and adjust the intervals until they disappear. The major third in particular should stand somewhat lower than in piano tuning.