Augmented, diminished and the tritone
Name intervals that are one semitone larger or smaller than usual, and recognise the tritone.
Major, minor and perfect cover most intervals. Two more qualities cover the rest: one semitone larger, and one semitone smaller.
Augmented and diminished
- An augmented (A) interval is one semitone larger than a major or perfect interval: C–F♯ is an A4; C–G♯ is an A5.
- A diminished (d) interval is one semitone smaller than a minor or perfect interval: C–G♭ is a d5; C–B𝄫 is a d7.
The letters decide the number
Two intervals can have the same size and different names, because the letter names decide the number. These pairs are enharmonic intervals: they sound the same in isolation but behave differently in context. Chords and scales need the correct name (Modules 7 and 10).
| Interval | Name | Same size | Name | Semitones |
|---|---|---|---|---|
| C–G♯ | A5 | C–A♭ | m6 | 8 |
| C–D♯ | A2 | C–E♭ | m3 | 3 |
| C–B𝄫 | d7 | C–A | M6 | 9 |
| C–F♯ | A4 | C–G♭ | d5 | 6 |
The tritone
The tritone is 6 semitones: three whole tones. It is an A4 or a d5, depending on the spelling. It divides the octave exactly in half, and inverted it is still a tritone. In a major scale it occurs once, between the 4th and the 7th degree: F–B in C major.
The tritone sounds unstable, and it “wants” to resolve. That pull is the engine of the dominant seventh chord, which you will meet in Module 10.
Play F (fret 3 on the D string) and B (fret 4 on the G string) together: a tritone. Then move F down to E (fret 2 on the D string) and B up to C (fret 5 on the G string). The tension resolves.