Every interval from unison to octave — how many semitones, what to call it, a well-known song that opens with it, its inversion, and a button to hear it.
| Interval | Semi. | Reference song (ascending) | Hear |
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An interval is the distance between two notes. Its size — second, third, fourth — counts letter names; its quality — major, minor, perfect, diminished, augmented — fine-tunes that by a semitone. A perfect fifth is 7 semitones and sounds stable; a tritone is 6 and sounds like it needs to move.
The reference-song trick is how most musicians learn intervals by ear: the first two notes of a tune you already know become a permanent handle for that distance. Once a few are automatic, drop the songs and test yourself in the Interval Ear Trainer.
Intervals are the building blocks of everything above them: stack two thirds and you have a triad, which the Chord Library covers; fill the octave with a pattern of tones and semitones and you have a scale, in the Scale Library. The Ear Training course takes it from here.