In a song, to add interest and variety changing key, or modulation is a great tool to use.
Songs typically modulate from a Tonic key to Dominant(V), Tonic Key to Subdominant(IV), or Tonic to Relative Minor.
Wait…..what does this mean?
In the key of C major, C is the tonic key or main key. It can modulate to F major (Subdominant), or to G major (Dominant) or to A minor (relative minor).
If you look at the Circle of Fifths, you can see that in the case of C major, for example, G major, F major and Am are the closet chords to C. This means they have many notes that are similar and therefore are the easiest to modulate to.
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