How Secondary Dominants Act as Key Changes on the Guitar
A secondary dominant is really just an altered version of an existing chord. This change to the chord is to create a dominant 7th. What gives it its secondary dominant function is that it leads to another chord a 5th below. So the I chord is altered to I7 to create movement toward IV, the ii chord is altered to II7 to highlight V, and so on.
In this way, secondary dominants are like mini key changes.
Think of the chord progression in C major where G7 leads to C. This is a common V7-I progression in C major. But what if these two chords appeared in F major, as in the progression F-Dm-G7-C? Although the progression as a whole is in F major (I-vi-II7-V), the G7 chord has notes in it that don’t belong to this key but instead belong to C major.
For your solos to work in this chord progression, you have to start out in F major and then switch patterns to C major when you reach the G7 chord. Finally, when the C chord sounds, you have to switch back to F major patterns.
When playing over a secondary dominant, you need to switch to its parent major scale. Switching scales over secondary dominants can be quite a challenge, but fortunately, making the switch isn’t always necessary. With the right note selection, you can stay in one scale and avoid any troublesome notes.
Here is an example of the song Aura Lee, an old American Civil War song that Elvis Presley adapted for Love Me Tender. The traditional version has several secondary dominants. Notice that the melody remains in the key of C even though secondary dominants appear in the progression. The song uses an F note over an A7 chord even though the F isn’t part of the A dominant scale.