Music Theory for the 21st-Century Classroom

In this chapter and the next, we will study tonicization, which means treating a chord other than the $\text{I}$ chord like a tonic by approaching it with its dominant. In diatonic harmony, the $\text{V}$ chord (the dominant) resolves to the $\text{I}$ chord (the tonic). A secondary dominant is a major triad or dominant seventh chord that resolves to (or tonicizes) a chord other than the $\text{I}$ chord.

Sing the bass line of the example above and notice that a secondary chord, through its chromaticism, intensifies the drive to the next chord.

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Principle 17.2.2. Secondary Dominants.

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The Roman numeral after the slash is the chord being tonicized by the $\text{V}$ chord before the slash.

You may find that you want to analyze the ${\text{D}}^7/{\text{F}}^{♯}$ in the example above as a ${\text{II}}_5^6$ instead of a ${\text{V}}_5^6/\text{V}$ (which we pronounce as “${\text{V}}_5^6$ of $\text{V}$”), and the ${\text{E}}^7/{\text{G}}^{♯}$ as a ${\text{III}}_5^6$ instead of ${\text{V}}_5^6/\text{vi}$ (“${\text{V}}_5^6$ of $\text{vi}$”). Notice, however, that a $\text{ii}$ chord is typically minor in a major key and diminished in a minor key (${\text{ii}}^{\circ }$), making uppercase $\text{II}$ a chromatic harmony for which the proper label is $\text{V}/\text{V}$.

While labeling ${\text{D}}^7$ as ${\text{II}}^7$ in C major makes the root clear, it does not communicate the function of the ${\text{D}}^7$, which is to progress to a G major chord (the $\text{V}$ chord, or the dominant in C major).

Also, notice that ${\text{vii}}^{\circ }$ is not tonicized with its secondary dominant in the example above. Listen to the following example to understand why diminished chords such as ${\text{vii}}^{\circ }$ and ${\text{ii}}^{\circ }$ in minor are not tonicized.