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7th Chords and Music Dissonance
When it comes to music dissonance, it all comes down to 7th chords, at least in harmony. All 7th chords are dissonant, and the 7th chord is the only other main chord type besides the triad type, which is consonant.
For example, the C major chord consists of the notes C, E, G, which is made up of two consonant intervals, C – E (a major third), and E – G (a minor third). If you’re going to stack on another third interval, it can only be either G – B♭ (a minor third interval), or G – B (a major third).
In either case, two things apply:
- The last remaining note of the chord is either a flatted 7th of the scale (B♭ in this example), or a natural 7th (B). Which is why this type of chord is called a 7th.
- Whether the note you add is a flatted 7th (B♭) or a natural 7th, (B), this added note will always be dissonant because it’s either a whole tone or a semitone removed from the tonic note (C, in this example).
And as you found out in Chapter 4, whole tone and semitone intervals are always dissonant (Table 36).
If even one interval of a chord is dissonant (internal or outer), the whole chord is dissonant. That’s why all chords except major and minor triads are dissonant.