Chords from Outside the Scale
Many songs have chords that use notes that aren’t in the scale of the rest of the song. There are many ways of doing this, some more common than others.
Using Chords from the Minor Scale
One way to do this without it sounding too random is to use a chord from the minor scale when the song is in a major key. A song that’s in C major might have a chord from C minor. Try playing this chord progression: C, F, Fm, C. You might recognize this sound. All the chords are from the C major scale except the Fm. Using the Fm draws the listener to the note that’s not in the scale, the Ab>, which is the third of Fm. If you play the whole chord progression, you might notice your ear is drawn to the A in the F chord, which moves to the Ab> in the Fm chord, then moves to the G in the C chord. It’s like there is a melody within this chord progression that starts on the A and moves down one semitone at a time. Experiment with adding other chords from the minor scale to your songs and improvisations.
The Dominant Seventh Chord
Another common way to use chords from outside the scale is with the dominant seventh chord. This is the chord that’s a major chord with a minor seventh. I already explained that in the key of C major, a G7 chord creates a strong feeling that the next chord should be C. Likewise, an A7 chord would create a strong feeling that the next chord should be D. As a general rule, dominant seven chords want the root of the next chord to be a fifth lower.
So, in the key of C major. you might have a chord progression that goes like this: C, G, C. We could add a seventh to the G chord to make it a dominant seventh chord: C, G7, C. We’re still in the C major scale here. But we could add another dominant seventh chord that leads into the G7. This would be D7, because D is a fifth higher than G. So, the chord progression becomes C, D7, G7, C. D7 is not in the C major scale, but it still sounds fairly normal in this case because it leads into the G7 chord. You could do this to any chord in the C major scale. For example C, E7, Am, F. In this progression the E7 leads into the Am because E is a fifth above A.