A specific interval has a number and a quality: major third, perfect fifth or diminished fourth, for example. First count the generic interval from the letter names. Then count the semitones to find its quality.
A reference within one octave
This table gives the perfect, major and minor intervals, measured upwards from C. Each example uses the nearest indicated note at or above the starting C.
| Interval | Semitones | Example |
|---|---|---|
| Perfect unison | 0 | C to the same C |
| Minor second | 1 | C up to D flat |
| Major second | 2 | C up to D |
| Minor third | 3 | C up to E flat |
| Major third | 4 | C up to E |
| Perfect fourth | 5 | C up to F |
| Perfect fifth | 7 | C up to G |
| Minor sixth | 8 | C up to A flat |
| Major sixth | 9 | C up to A |
| Minor seventh | 10 | C up to B flat |
| Major seventh | 11 | C up to B |
| Perfect octave | 12 | C up to the next C |
Unisons, fourths, fifths and octaves belong to the perfect family. Seconds, thirds, sixths and sevenths can be major or minor. A minor interval is one semitone narrower than the major interval with the same number.
Augmented and diminished intervals
For the same letter count, widening a perfect or major interval by one semitone makes it augmented. Narrowing a perfect or minor interval by one semitone makes it diminished.
| Notes, ascending within an octave | Letter count | Semitones | Interval |
|---|---|---|---|
| C to F | Fourth | 5 | Perfect fourth |
| C to F sharp | Fourth | 6 | Augmented fourth |
| C to G flat | Fifth | 6 | Diminished fifth |
| C to F flat | Fourth | 4 | Diminished fourth |
The two six-semitone intervals are both tritones. F sharp and G flat occupy the same fret in equal temperament, but their spellings produce different interval names. Four semitones can likewise mean C to E, a major third, or C to F flat, a diminished fourth.
Try a third on one string
In standard tuning, string 2 fret 1 is C. Play it followed by fret 4, E flat: three fret steps give a minor third. Follow C with fret 5, E, for a major third of four semitones. Count the moves between frets; do not count the starting fret as a semitone.
Next, try inverting those intervals.
Reference: Robert Hutchinson’s interval identification and augmented and diminished intervals.