Do dominants, secondary dominants, or tritone substitutions have to be seventh chords?
You might think yes. But really, no….
This deserves further explanation which I provide in my response to the Quora poster.
Included in my explanation is how even single notes can imply the dominant effect. And a deeper dive into the all-important tritone.
I also touch on how and why the tritone doesn’t always imply a dominant effect either. (Hint: Think minor six chords or a simple IV chord.) And why the Blues totally drives he classical theorists crazy.
The reason seventh chords are used so often setting up a resolution to the tonic, is that they have two drivers of the need to resolve. Can you guess what the two drivers are?