Every Major Key has a Minor Key that shares the same notes with it.
How does this work, how can we find our Relative Keys, and what can we do with it? Let's check it out!
To see this for ourselves, let's play through the C Major Scale by playing our Major Scale, starting from the 8th fret of the low E.
That will produce the following notes:
C-D-E-F-G-A-B.
If you haven't noticed by now, the C Major Scale is the easiest of the Majors to work with and talk about, because it doesn't contain any sharps or flats.
Now, let's play through our Minor Scale, starting it on the 5th fret of the low E.
This will produce an A Major Scale for us.
Have a look at the notes produced:
A-B-C-D-E-F-G.
It's the same seven notes as the C Major scale!
The only difference is where the Root Note resides.
As you can see, we can easily find the "Relative Minor" of any Major Scale simply by starting it three frets lower.
The opposite is true for finding a Relative Major Scale.
For example, if you're in G Minor, and you want to find the Relative Major, move your Root Note three frets higher.
That will move us from G, up to G#, A, and finally to A#. That's it, we found it. The Relative Major of G Minor is A# Major.
You can play the Major Scale starting on the 6th fret of the low E to check.
Now, how can we put this to use?
From a songwriting perspective, you can swap to the relative key for a change in mood, without actually changing the notes that are being played.
For example, you could move from a chord progression treating C as the Root Note, to one treating A Minor as the Root Note. During this change - you could keep the melody being played exactly the same. This is an extremely powerful emotional tool.
From an improv/melody perspective, this means that any shapes you've learned (chords, scales, arpeggios, etc.) can all be used in both Major and Minor keys, provided you place them in the right spot.
For example, you can solo with a Minor Pentatonic over a Major chord progression. You can use a Major Arpeggio Shape over a Minor Progression. In a sense, it's doubled your options. You'll be able to provide a ton of character to your playing in this way!
Understanding Relative Keys will start to open doors of possibility for you that you may not have even recognized before now!
Spending some time applying Major shapes over Minor Keys (and vice versa) will make this new option second-nature for you.
Try it out for yourself!