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How a Capo Changes Key

Question by Anon

I wrote my first song on guitar. I put the capo on the third fret and then I used the chord shapes for G, Am, Em, and C/B.

I am trying to determine what key this would be, so that I can start working on a melody using notes from the appropriate key.

Answer

Firstly, congratulations on writing your first song!

In short, the new key would be Bb (B flat) major, so the natural scale to build a melody over this progression would be the Bb major scale.

Let's break it down (if you're interested in the theory)...

If there were no capo, the key would be G major.

This is because all the chords/notes being played would be from the G major scale (you can learn about using this scale to determine and transpose/change key in the chord progressions section)...

Tonic234567
GAmBmCDEmF#dim

So the sequence is I - ii - vi - IV (or 1, 2, 6, 4) from the G major scale.

With the capo on the third fret, that's the equivalent of moving everything up by three semitones (the equivalent of three frets).

Start by moving the tonic chord (G in this case) up three semitones (three positions in the musical alphabet).

You can use the table below to see what the tonic chord becomes with the capo on each fret (I've highlighted the 3rd fret - G becomes Bb)...

Open Tonic1st2nd3rd4th5th6th7th8th9th10th11th
EFF#GAbABbBCC#DEb
GAbABbBCC#DEbEFF#
ABbBCC#DEbEFF#GAb
CC#DEbEFF#GAbABbB
DEbEFF#GAbABbBCC#

So our open G shape becomes a Bb major chord.

We can now use the major key scale of that new tonic to determine what the other chords in that key will become.

Again, the below table will help you with this (in this case, find Bb and they are the new chords for that key)...

Tonic234567
ABmC#mDEF#mG#dim
BbCmDmEbFGmAdim
BC#mD#mEF#G#mA#dim
CDmEmFGAmBdim
C#D#mE#mF#G#A#mB#dim
DEmF#mGABmC#dim
EbFmGmAbBbCmDdim
EF#mG#mABC#mD#dim
FGmAmBbCDmEdim
F#G#mA#mBC#D#mE#dim
GAmBmCDEmF#dim
AbBbmCmDbEbFmGdim


So our 1, 2, 6, 4 progression in the key of Bb major will be...

Bb / Cm / Gm / Eb

As you're using a capo, you'll already be playing the right chord shapes for this progression.

Am becomes Cm
Em becomes Gm
C/B becomes Eb/D

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