Just intonation can be contrasted and compared with equal temperament, which dominates Western instruments of fixed pitch and default MIDI tuning. In equal temperament, all notes are defined as multiples of the same basic interval. Two notes separated by the same number of steps always have exactly the same frequency ratio. However, except for doubled frequencies (octaves), no other intervals are exact ratios of integers. Each just interval differs a different amount from its analogous, equally tempered interval.According to my teacher, the spoiler here is the piano, which uses the equal temperament, but when singers or string quartets get together without a piano in the mix they sometimes use Just intonation rather than Equal temperament, which is now the standard.
And now for something completely different - I admit I LOL'd when I suddenly saw this on the news stand at the deli:
