Why are these parallel 9ths okay?

November 19, 2022

In my teaching, I try to avoid fixing student work by writing out a corrected version. Instead, I try to clearly mark every element that's problematic and do my best to explain why each thing has been marked, and what could be done to correct each problem. This way students are led to their own solutions. Sometimes though, to save time, particularly if we've been working on something for quite a while and problems are still cropping up, I might go ahead and just show something by writing out a new version for the student to compare with their version. Last week I did that for one of my students who is working on his first fugue expositions, and it so happened that what I wrote included the following measure:

The student then pointed out that this measure contains parallel 9ths. Did you notice them?

He recalled that some time ago I had pointed out parallel 9ths in something he'd written as being a problem. Apparently, these parallel 9ths are okay, but why? A valid question. I explained it this way … The underlying voice leading is parallel 10ths. We can start there and work our way toward the version with the parallel 9ths:

The F-sharp in the bass can be held as a suspension. The dissonant 9th sounds nice, and the F-sharp then sounds like it must move to the E to resolve the dissonance.

To further accent this dissonance while making the lower part more active, the suspension can be elaborated with an upper neighbour. This is where our parallel 9ths come in.

Parallel dissonances of this kind are a fairly common occurrence in Bach's music. Parallel 7ths and 2nds aren't entirely rare, since they result from this technique of emphasising dissonance and keeping voices rhythmically active — which is in fact one of the defining characteristics of Bach's music. Like everything else in this art form, there are a lot more ways to do it wrong than there are ways to do it right!

Best Regards,
AAH