An interval is the distance between two notes. Name intervals and you can describe any melody or chord in a few words: a melody that leaps a fifth, a chord with a minor third in it.
Counting in semitones
The smallest step on a keyboard, one key to the very next, is a semitone. Two semitones make a whole tone, such as a major second. Every interval inside the octave can be counted this way.
| Semitones | Name | From C to |
|---|---|---|
| 0 | Unison | C |
| 1 | Minor second | D♭ |
| 2 | Major second | D |
| 3 | Minor third | E♭ |
| 4 | Major third | E |
| 5 | Perfect fourth | F |
| 6 | Tritone (augmented fourth) | F♯ |
| 7 | Perfect fifth | G |
| 8 | Minor sixth | A♭ |
| 9 | Major sixth | A |
| 10 | Minor seventh | B♭ |
| 11 | Major seventh | B |
| 12 | Octave | C |
The tritone is the odd one: three whole tones, six semitones, exactly half an octave.
Ratios: why some intervals sound settled
Two notes an octave apart have a frequency ratio of 2:1. In just intonation the main intervals are small whole-number ratios: 3:2 for the perfect fifth, 4:3 for the perfect fourth, 5:4 for the major third, 6:5 for the minor third, 5:3 for the major sixth. The simpler the ratio, the more settled the interval tends to sound. Lesson 12 explains why a modern keyboard bends these ratios very slightly.
Intervals are also measured in cents: the octave is divided into 1,200 equal parts on a logarithmic scale, so an equal-tempered semitone is 100 cents.
Wider than an octave
Intervals larger than an octave are called compound intervals. A ninth is an octave plus a second, a tenth an octave plus a third. They matter in chords: the "9" in a C9 chord is a D an octave and a step above the root.
Sources
Every date, name, tempo and origin on this page comes from these pages, read on 2026-10-24.