Watch the Modulating online guitar lesson by Matthieu Brandt from Songwriting on Guitar
In previous sections we've looked at introducing chords from different keys, straying away from the original key your song starts out with. This type of chord borrowing is very common in pop and rock music and our ear has gotten used to hearing them.
Another way of adding more possible chords is to 'modulate' meaning to use a different chord as the tonal center of a song section. This will change the mood of that section and will make the ear perk up.
The most common modulation uses the 'relative' relationship.
The chords we generally use in our compositions use tertiary harmony. When you use this harmony, the chords derived from the Ionian major scale and the scale that starts 3 half notes lower are the same. In your composition you can chose to centre a section around one of these chords: the major chord built on the tonic of the Ionian major scale OR the tonic of the Aeolian minor scale, three half notes down.
Switching between these tonal centres will create contrasting moods.