Section 22.4 Modulations with Diatonic Pivot Chords
Modulations with pivot chords will be analyzed using a pivot bracket, as we've seen earlier in the chapter.
In a diatonic common chord modulation, the pivot chords will be diatonic in both keys.
Subsection 22.4.1 Determining Common Chords Between Keys
In order to compose a diatonic common chord modulation, you need to determine which chords are diatonic—having the same root and quality—in both keys.
We can repeat this process with Roman numerals, aligning the roots of the chords in the two keys.
If you are a composer wanting to write a diatonic common chord modulation, you need to determine the best place within a progression to pivot to the new key. To do this, you need to consider the harmonic function of the pivot chords.
Subsection 22.4.2 Harmonic Functions of Diatonic Pivot Chords
In terms of harmonic function, composers typically do not use a pivot chord that has dominant function in the new key because such a modulation might sound abrupt and unconvincing. Instead, the pivot chord in the first key often has tonic or tonic prolongation function.
In the following example, the pivot chord simultaneously has tonic function in the first key and pre-dominant function in the second key. This creates a more seamless and less jarring progression to the second key.
The pivot chord in the first key, G, has tonic harmonic function, while in the second key, D major, the G chord has pre-dominant function.
Observe the harmonic function of the pivot chords in the following examples from Bach chorales.