Secondary Dominant

When we talked about the dominant chords, we discussed that the dominant resolves to the tonic: the dominant chord creates a tension, a feeling of unfinished melody, which pulls the melody into its tonic. For example, for a melody written in C major key, the dominant chord G major will want to resolve to the tonic C major.

If you forgot: the dominant chord is built on the fifth position (fifth scale degree) of a scale. In C major, the positions are C D E F G ..., so the dominant is G major chord.

Now, the following thought brings us to the concept of secondary dominant: if the dominant chord resolves to its tonic:

G major → C major

It's hard to hear the tension or pulling just by having chord buttons and no melody; bear with me! I'll have a great example for you.

then can we continue this to the left to have some chord that would resolve to G major, which in turn would resolve to C major?

If we imagine that G major is our tonic, the dominant in this “temporary” tonic will be D major (fifth position in the G major scale: G A B C D ...), so D major will “pull” to G major:

D major → G major

But in C major, G major pulls to D major, which gives us

D major → G major → C major

So D major is the dominant to the dominant, which makes it a secondary dominant.

You probably don't hear any specific tension or pulling just by pressing D, G, and C, so what is this all about?

Secondary dominant in the major key

Here's “The Entertainer” by Scott Joplin; I'll give you a simplified slow version. It's written in C major, and the first phrase is just the tonic, the subdominant, and the dominant. First, please try to play the chords without any hints. Remember that the subdominant develops the melody, and the dominant pulls it back to tonic. I'll show you the answer below:

Here is the same first phrase with the marked chords. C major is the tonic chord, F major is the subdominant chord, and G major is the dominant:

This should be pretty straightforward. But the second phrase introduces a new chord, and I'll show it:

Have you noticed this F♯ in the melody, which perfectly matches the D major chord (D F♯ A), and how naturally this D major chord resolves to G major? This is how the secondary dominant works.

Secondary dominant in the minor key

For the minor key example, let's look at “Those Were the Days” again: amazing song, shows so many different harmonization techniques! The first phrase is something we already figured out: it uses the parallel major chord (A major, in the song written in A minor). Try playing it without hints first:

Now, just to check yourself, here are the hints. The tonic chord is A minor, the subdominant is D minor, the dominant (E major) is not used in this fragment, but we do have the parallel major chord A majir:

But let's see what happens next!

Notice how the B major chord appears from nowhere, pulling the melody to its dominant E major, which, in turn, will very soon resolve to A minor when the chorus begins in the next measure. This B major chord is the dominant chord for E major, so it's the secondary dominant chord for A minor.

Summary

The secondary dominant chord is a major chord built on the second position (second scale degree) of scale (major or minor). It resolves to the dominant chord (built on the fifth position), which, in turn, resolves to the tonic chord (first position).

In C major: D major → G major → C major

In A minor: B major → E major → A minor

I hope it all makes sense so far!