Secondary Dominant – Definition

Let’s dissect the definition of a secondary dominant to fully understand it.

Definition: A secondary dominant is a dominant functioning chord of any major or minor chord in a key other than tonic.

A secondary dominant is . . .

a Dominant Functioning Chord . . .

Our definition starts “A secondary dominant is a dominant functioning chord . . .” What is a dominant functioning chord?

In our discussion of harmonic progressions, we talked about Tonic (T), Predominant (PD), and Dominant (D) functions. Our final diagram had chords in at last one of these three categories:

For secondary dominants, the dominant functioning chords are
V, V7, vii^o, vii^ø7, and vii^o7

. . . of any major or minor chord in the key . . .

In our example from “Easter Parade,” there was a V7/V in the last line of the example:

The C7 is a V7 in the key of F – the 5th of Bb. The C7 chord makes the F chord which follows a temporary tonic; the C7 tonicizes the F (V) chord.

Since the chord that follows the secondary dominant becomes a temporary tonic, it must be a major or minor triad since these are the only chords that can serve as a tonic. We have no scales for which a diminished and augmented triad is tonic.*

Therefore, the chord that is being tonicized must be
a major or minor chord in the key:
In major keys: ii, iii, IV, V, vi
In minor keys: III, iv, V, VI

. . . other than tonic.

If the written dominant chord naturally resolves to the tonic of the key, then the chord is the primary dominant and not a secondary dominant.

Summary

You can think of a secondary dominant as
an altered chord that follows this format: X/Y
where

X = V, V7, vii^o, vii^ø7, and vii^o7
Y = major or minor triad in the key other than I

A secondary dominant tonicizes the chord that follows it (the Y).

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*Some may argue that an octatonic scale has a diminished traid (or seventh chord) built on the final of the scale, but there is no dominant to tonic function in this scale.