Secondary dominant chords fall into a larger category of harmony called chromatic harmony.
Under the topic of harmony, there are two large categories: diatonic and chromatic. Diatonic harmony refers to only the chords that are part of the key you’re in. For example, if you’re in the key of A major, the diatonic chords are: A major, B minor, C♯ minor, D major, E major, F♯ minor, and G♯ diminished.
Chromatic harmony, on the other hand, refers to any chords that are not part of the key you’re in. There are pretty infinite options for this, but some common chromatic chords that are used while you’re in the key of A major are B dominant 7 that leads to E major, B♭ major, and D minor.
Studying chromatic harmony in a more traditional way gives you a way to organize all of the different types of chromatic harmony and the effects they each have on listeners. There are different ways to change keys: some are subtle, some are sudden and dramatic. There are different ways to borrow chords for different effects. There are different ways to use what are called secondary functions to do what’s called tonicize a diatonic chord.
What Are Secondary Dominant Chords?
Tonicizing means to make a chord sound stronger than it ordinarily would be in its solely diatonic context. That means that secondary dominant chords, which don’t belong in the key, make chords that do belong in the key sound stronger.
Secondary dominant chords are chords that have roots that are a 5th away from the chord they tonicize, and they serve a dominant function. This means they often come in the form of a dominant 7th chord (major-minor 7th chord).
In the key of C major, for example, if we wanted to tonicize the G major (V) chord, we would use G major’s dominant chord as a secondary dominant. G major’s dominant is D major, which is spelled D – F♯ – A. Because C major has no sharps or flats in it, you can see that D major is a chromatic chord that leads strongly to G major.
In a Roman numeral analysis, we call that a “V of V,” which is notated as V/V.
It’s easier to see the effect that secondary dominant chords have in the context of an actual song, and “Don’t Know Why” by Jesse Harris (famously covered by Norah Jones) has some good examples.
Secondary Dominant Chords in “Don’t Know Why”
To show you the effect that secondary dominant chords have, let’s start off with a wholly diatonic version of an excerpt of the song.
It doesn’t sound bad. It’s still quite pleasant and would make a popular song.
The cool thing, though, is that we can add in chromatic chords, specifically secondary dominant chords, just by changing single notes. The chromatic chords are bolded and colored in red in the example below.
The first is B♭7. Because we’re in the key of B♭ major, the seventh chord built on B♭ would diatonically be a B♭ major 7, not a dominant 7th. That, alone, signals that something chromatic is going on. We get an A♭ in the middle voice, which changes the chord from being B♭ major 7 to B♭ dominant 7.
If we look to the next chord, E♭ major, you’ll notice that B♭ has the relationship of a 5th above E♭. If E♭ were the tonic of the key, then B♭ dominant 7 would be its dominant V chord. But because we’re not in that key of E♭ major, but we get a B♭7, that means that E♭ major is being tonicized by B♭7.
B♭7 is a secondary dominant!
It provides forward momentum and a small sense of tension and release to lead us to that E♭ major, which is the IV chord in the key of B♭ major. That means that the Roman numeral analysis would call the B♭7 a “V7 of IV,” written as V7/IV.
Moving forward, we also get a borrowed augmented chord: D+. We’re going to skip over this, as it’s a whole other chromatic topic.
The final chromatic chord is the C7, another dominant chord that doesn’t belong in the key of B♭ major! Diatonically, the chord built on C in the key of B♭ major would be C minor.
To get this chromatic chord, all you have to do is raise the E♭ to be an E♮, which you can see in the middle voice in this sheet music. Again, because it’s a dominant seventh chord, it likely tonicizes the chord after it.
F7, which is the V7 of B♭ major, is the next chord and is a 5th below C7. C7 is tonicizing F7 here, so it’s a secondary dominant chord.
This particular instance of having a secondary dominant go to the diatonic dominant chord and then the tonic of the key is a very common and strong way to end a phrase. V7/V7 V7 I.
Give it a listen in the video below to get the full sense of the differences between the diatonic and chromatic versions of this song.
To learn more about secondary dominants and many other topics in chromatic harmony (4 ways to change keys, secondary leading tone chords, borrowed chords, flat 6, Neapolitan chords, augmented 6ths, etc.), check out the Chromatic Harmony: Advanced Chord Progressions eBook and the Chord Progressions: Cadences to Key Changes course!