Sometimes a song is written in a key that isn’t suitable for a singer’s range.
We can use our knowledge of intervals to put the song into a different key.
This is called transposition.
If a song is in the key of C major and it is too low, it can be played in a higher key such as D major.
First of all, find the interval between the original key and the new key.
The interval between C and D is a major 2nd.
To transpose from C major into D major, all notes and chords will get raised by a major 2nd interval (2 semitones).
So if the original chord progression in C major is:
C G Amin F
The new chord progression in D major will be:
D A Bmin G
If a song is in G major and I want to put it into C major, then I will need to raise it by a Perfect 4th interval. This is because the difference between G and C is a Perfect 4th.
So if the original chord progression in G major is:
G Amin C D
The new chord progression in C major will be:
C Dmin F G
Many guitar players use a capo to transpose when it is not really necessary, like for the above example. If you know your intervals you can easily transpose the chords to a new key.
It is best to only use a capo if you find barre chords difficult or if you wish to create different tones by playing chords higher up the neck.