Understanding Secondary Dominants

We introduced chromatic harmony in this music theory lesson. Simply stated, a chromatic chord is any chord that occurs outside of a given key.

As we’ve discussed in several other music theory lessons, the dominant-tonic (V-I) harmonic progression is one of the strongest progressions in all of music. Tonal music seems to have a strong tendency to move down by fifth. Because of this tendency, composers have used chromatic chords called secondary dominants, which are dominant chords borrowed from other keys. The concept is easier to demonstrate than to explain:

Here is a V7-I harmonic progression in G major. (The actual chords are D7 – G:)

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Here is a basic I-IV-V-I harmonic progression in C major:

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Now, we’ll insert the V7 chord (D7) from G major before the V chord (G) in the basic progression in C major:

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We took a chord that belongs in G major and put it in a progression in C major. Since that D7 chord is the dominant seventh (V7) of G we call it a V7/V (“five-seven of five”), or a secondary dominant. We have to add the F-sharp accidental because we did not change key signatures, we simply inserted a chromatic chord.

Do you see why it’s called a secondary dominant? The D7 chord functions as a dominant seventh of G, but the key is still C major (where G is the primary dominant). It’s as if, for an instant, the key changes to G major, but we don’t call it a key change because it moves by so quickly, and the progression still ends in C major. We use the term tonicize to describe the function of secondary dominants. For example, in the progression above, the G major chord (which happens to be V in C major) is tonicized by it’s dominant seventh chord (D7).

A secondary dominant may be a triad or a seventh chord. But remember, as we studied in diatonic harmony, the dominant triad is a major chord; the dominant seventh chord is a major-minor seventh chord. Therefore, we must add any necessary accidentals to make the secondary dominant either a major chord or a major-minor seventh chord. In the example above we added a sharp to the F to make the chord a major-minor seventh.

Let’s look at another example:

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In the example above, we tonicized the IV chord. In other words, we inserted the dominant seventh of F before the IV chord, thus creating a V7/IV (“five-seven of four”).

Remember, the secondary dominant has to be the dominant chord from the key of the chord that is being tonicized. Above, we wanted to tonicize the F chord. So we ask, what is the dominant of F? Thinking in the key of F for a moment, go to the dominant scale degree (C) and spell either a major or a major-minor seventh chord. Since the key of F has a B-flat, the secondary dominant must have a B-flat in order to be a major-minor seventh chord.

Secondary dominants work just like primary dominants: they may be triads or seventh chords, and they may be in any inversion.

What Chords May Be Tonicized?

Any major or minor chord may be tonicized. Here are examples of all the chords that can be tonicized in C major and A minor:

Notice that there is no secondary dominant for the leading tone chord in a major key (V/vii) and no secondary dominant for the supertonic chord in a minor key (V/ii). Why is this? It’s because these chords are diminished triads (vii° and ii°). You cannot tonicize diminished chords, because there are no diminished keys. For instance, a key such as C-diminished does not exist.

In the next lesson, we’ll look at secondary dominants in actual music and show you some typical chord progressions using secondary dominants.