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The problem it solves
When you see a key signature you can't tell at once whether the music is major or minor: the same signature serves two keys. Knowing the relative pair helps you not confuse C major with A minor and to find the real tonal centre.
Detailed theory
Key idea
A major key and its relative minor share the key signature and the notes, but have different tonics (centres).
The relative minor is the 6th degree of the major scale: a minor third (3 semitones) below the major tonic.
Understand it
Two keys are relative when they share the same key signature: exactly the same notes, the same sharps or flats. The difference is not the material but which note is the tonic, that is, where the centre of gravity lies. That is why C major and A minor are relative: both use only the white keys, but one revolves around C and the other around A.
The relative minor always falls on the 6th degree of the major scale. In C major the 6th degree is A, so A minor is the relative minor of C major. The same happens with G major and E minor (they share F#) or F major and D minor (they share Bb): the minor is always born from the 6th degree of the major.
A quick way to find it is to count: go down a minor third (3 semitones) from the major tonic and you reach the tonic of the relative minor. Conversely, if you go up a minor third from the minor tonic you find the relative major again. In fact, the natural minor scale of the relative minor is simply the major scale started from its 6th degree.
Think of a family photo: it is the same people (the same notes), but depending on who stands in the centre of the portrait the whole meaning of the image changes. When C is in the centre you hear a bright key; when A is in the centre the same material turns darker and more inward. The colour changes because the centre changes, not because the notes change.
This relationship is the basis for reading key signatures, transposing songs and moving between the major and the minor mode without changing notes. Recognising the relative pair lets you immediately guess the two possible keys a signature hides.
Staff & keyboard
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The two tonics marked within the same material: C (centre of the major) and A (centre of the relative minor).
How to recognise it
How it's written
Faced with a key signature, remember it can belong to two relative keys: the major and its minor on the 6th degree. Look at which note acts as the centre (where it begins and ends, where the music rests) to decide whether you read in major or in its relative minor.
How it feels
Play the white-key scale ending on C and then the same one ending on A: the material is identical, but the centre changes and with it the colour (C brighter, A darker and more inward). What you hear as different is not the notes but where they gravitate.
Common mistake
Confusing relative with parallel: the relative shares the key signature but changes tonic (C major ↔ A minor); the parallel shares the tonic but changes key signature (C major ↔ C minor).
Looking for the relative minor a third too far up or down: it is exactly a minor third (3 semitones) below the major tonic, or its 6th degree.
Try it
Start from C major and go down a minor third (C → A): you get A minor, its relative, with the same notes.
Play the white keys from C to C and then from A to A and notice that the material is the same; only the note the music rests on changes.
On the instrument
Staff & keyboard
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The C major scale: white keys that open and close on C, the tonic.
Staff & keyboard
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The A minor scale: the SAME notes, but now the tonic is A. Same material, different centre.
Interval distance
From A to C is a minor third (3 semitones): the distance separating the minor tonic from its relative major tonic.
Where it's used
- Reading key signatures
- Faced with a key signature, working out the two possible keys: the major and its relative minor.
- Finding the relative minor
- Going down a minor third from the major tonic (or to the 6th degree) to identify the relative minor key.
- Moving between major and minor
- Changing colour without changing notes, moving from a major key to its relative minor.
Examples
Chord progression
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The I of C major (C-E-G) and the i of A minor (A-C-E): the same set of white-key notes, two different centres.
Mini test
Check that you've got it.
0/9 answeredQuestion 1/9
What do a major key and its relative minor share?