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The problem it solves
You often want to change the colour of a piece (make it darker or brighter) without moving the tonal centre. The parallel gives you the key: same resting note, different scale, and with it a whole new character.
Detailed theory
Key idea
Parallel keys share the tonic, but have different scales and key signatures.
The natural minor lowers the 3rd, 6th and 7th degree a semitone versus the parallel major; the lowered 3rd (E→Eb) is what creates the minor colour.
Understand it
Two keys are parallel when they revolve around the same tonic but use different scales. C major and C minor both rest on C, but C major uses white keys and C minor brings in Eb, Ab and Bb. The centre does not move; what changes is the material around it.
The change that defines the minor is lowering three degrees a semitone versus the major: the 3rd (E→Eb), the 6th (A→Ab) and the 7th (B→Bb). Of these three, the most decisive for the ear is the lowered third: going from E to Eb turns the chord and the scale from major to minor and darkens its colour.
You must not confuse parallel with relative. The relative shares the key signature and changes tonic (C major ↔ A minor, same notes). The parallel is just the opposite: it shares the tonic and changes the key signature (C major ↔ C minor, same centre, different notes). This distinction is key so as not to mix up the two relationships.
Think of the same house repainted in cooler colours: the structure and position (the tonic) do not move, but the mood changes completely. Composers and arrangers use this closeness to borrow chords from the parallel (modal interchange) and tint a major piece with touches of its minor, or the other way round.
That is why the parallel is so practical: it lets you change colour while keeping the home note, compare directly the effect of the major and minor third, and open the door to modal interchange without losing the tonal centre.
Staff & keyboard
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The C natural minor scale with the three degrees lowered versus the major: Eb (3rd), Ab (6th) and Bb (7th).
How to recognise it
How it's written
Compare the two key signatures on the same tonic: the minor version adds flats (in C minor: Eb, Ab, Bb) that lower the 3rd, 6th and 7th degree. If the tonic does not move but these alterations appear, you are looking at the parallel minor, not the relative.
How it feels
Play the C major chord (C-E-G) and then C minor (C-Eb-G): the bottom and top notes do not move, only the middle one drops (E→Eb), and the colour goes from bright to dark. That third is the audible mark of the major↔minor change.
Common mistake
Confusing parallel with relative: the parallel shares the tonic and changes the signature (C major ↔ C minor); the relative shares the signature and changes the tonic (C major ↔ A minor).
Thinking you only need to lower the third: the natural minor also lowers the 6th and 7th degree versus the parallel major.
Try it
Play C-E-G and then C-Eb-G without moving either C or G: you will hear how lowering E to Eb alone turns the major into minor.
Write C major and C minor one above the other and mark the three degrees that change: 3rd (E→Eb), 6th (A→Ab) and 7th (B→Bb).
On the instrument
Stacked triad
The C major triad: C-E-G. The major third (C-E) gives it its bright colour.
Stacked triad
The C minor triad: C-Eb-G. The lowered third (E→Eb) is the only change, and it already darkens the colour.
Interval distance
C-E: a major third (4 semitones), the major one. The minor version lowers this note to Eb.
Interval distance
C-Eb: a minor third (3 semitones), the one of the parallel minor. A semitone shorter than the major third.
Where it's used
- Changing colour
- Moving from major to minor (or back) over the same tonic to darken or brighten a piece without moving the centre.
- Modal interchange
- Borrowing chords from the parallel mode (borrowed chords) to colour a progression while keeping the key.
- Telling it from the relative
- Not confusing the parallel (same tonic, different signature) with the relative (same signature, different tonic).
Examples
Chord progression
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The C major triad and the C minor one, one after the other: same tonic, only the third drops (E→Eb).
Prepares you for
Mini test
Check that you've got it.
0/9 answeredQuestion 1/9
What do two parallel keys share?