Consonance and dissonance
Describe intervals as stable or tense and know the usual classification.
Some intervals sound calm and finished, others sound tense. This lesson names that difference, because bass lines and chords use it constantly.
Stable and tense
Consonant intervals sound stable, at rest. Dissonant intervals sound tense and seem to want to move; that movement is called resolution. Dissonance is not “bad”: music moves by building tension and releasing it. Later lessons use this idea for chords (Module 8) and bass lines (Module 9).
The usual classification
| Group | Intervals | Examples from C |
|---|---|---|
| Perfect consonances | unison, octave, perfect 5th; the perfect 4th usually counts here too, but is treated as a dissonance in some classical part-writing contexts | C–C, C–G |
| Imperfect consonances | major and minor 3rds, major and minor 6ths | C–E, C–A♭ |
| Dissonances | major and minor 2nds, major and minor 7ths, the tritone | C–D, C–B, C–F♯ |
A hint from the numbers
The simplest frequency ratios sound the most consonant: the octave is 2:1, the fifth 3:2, the fourth 4:3, the major third 5:4 and the minor third 6:5. Equal temperament (lesson 1.4) slightly alters all of them except the octave.
Play a root and its perfect 5th together: open E and B at fret 2 on the A string. Then a root and its minor 2nd: A at fret 5 on the E string and B♭ at fret 1 on the A string. Describe the difference in your own words.