An interval is consonant when its two notes sound stable and pleasant together. It is dissonant when they create tension that asks to be resolved. This classification is basic to harmony and counterpoint. For a review of the basics, see the guide to musical intervals.
Full chart: classification of all the intervals
| Interval | Half steps (UK: semitones) | Classification |
|---|---|---|
| Perfect unison | 0 | Perfect consonance |
| minor 2nd | 1 | Strong dissonance |
| Major 2nd | 2 | Strong dissonance |
| minor 3rd | 3 | Imperfect consonance |
| Major 3rd | 4 | Imperfect consonance |
| Perfect 4th | 5 | Perfect consonance |
| Tritone (Augmented 4th / Diminished 5th) | 6 | Semi-consonance (conditional dissonance) |
| Perfect 5th | 7 | Perfect consonance |
| minor 6th | 8 | Imperfect consonance |
| Major 6th | 9 | Imperfect consonance |
| minor 7th | 10 | Strong dissonance |
| Major 7th | 11 | Strong dissonance |
| Perfect octave | 12 | Perfect consonance |
Consonances
Perfect consonances
These are the most stable intervals: the unison, the perfect 4th, the perfect 5th and the perfect octave. They are called perfect because their quality is always "perfect" (never major or minor), and because their inversions are also perfect intervals (4th ↔ 5th, unison ↔ octave). They are also the first intervals of the natural harmonic series. In classical counterpoint, perfect consonances serve as points of rest.
One practical exception: the perfect 4th is treated as a dissonance when it appears between the bass and an upper voice. In that case it asks to be resolved, usually to a 3rd.
Imperfect consonances
These are the major 3rd, minor 3rd, major 6th and minor 6th. They sound pleasant and stable, but with more "color" than the perfect ones. Their inversion changes quality (major ↔ minor), and in counterpoint they are the most used intervals for creating warm harmony. Imperfect consonances are the building blocks of triads.
Semi-consonance: the tritone
The augmented 4th and the diminished 5th are the same interval (6 half steps) written in two different ways: the tritone. It divides the octave exactly in half and has maximum tension, so it asks for resolution. It has long been nicknamed diabolus in musica ("the devil in music") because of its instability.
Traditional theory books call this category semi-consonance (or conditional dissonance). Mind the enharmony: F♯–C (a diminished 5th) and G♭–C (an augmented 4th) sound the same and are the same 6-half-step tritone. Changing the spelling changes the name of the interval, not its sound.
Dissonances
Strong dissonances
These are the major 2nd, minor 2nd, major 7th and minor 7th. They create tension that asks for resolution, whatever the context. In classical counterpoint, dissonances are carefully prepared or approached and then resolved, usually by step. Far from being "bad", they drive tonal music forward: the dominant seventh chord (V7) contains a minor 7th and a tritone, and its resolution to the tonic is the strongest cadence in the key.
Conditional dissonances
These are the semi-consonances: the augmented 4th and the diminished 5th, that is, the tritone. However it is spelled (F♯–C or G♭–C), it sounds the same and asks for resolution. This double reading of the tritone is also the basis of many enharmonic modulations in Romantic music.
Exercises
To practice: consonant and dissonant intervals exercises.