Major keys are easy to understand, at least in classical music, because a major key and a major scale are coextensive. Minor keys are harder, because you can’t just equate them to particular minor scales. To understand how chords work in minor keys, I’m going to walk you through a standard progression that happens throughout the final movement of Bach’s Violin Partita No. 2 in D minor, the famous Chaconne.
Here’s the complete performance by Viktoria Mullova, with MIDI visualization in Ableton Live created by me:
Read more about the Chaconne and hear the Afro-Funk remix. There’s a lot there to dig into! But right now, I’m just going to talk about the first few measures. The opening phrase is four chords: Dm, Eø7 with its 7th in the bass, A7 with its 3rd in the bass, and Dm again.
Together, the chords form a ii-V-i in D minor. In the remainder of this post, I’m going to talk through these three chords and their associated scales in detail. Try them for yourself on the aQWERTYon.
The basic ingredient for the key of D minor is the D natural minor scale:
The tonic chord in D natural minor is Dm, unsurprisingly. I like the jazz convention of using lowercase Roman numerals for minor and diminished chords, so let’s call Dm the i chord:
The ii chord in D natural minor is E half-diminished, or, as jazz musicians call it, Em7b5. You can think of this chord as being generated by E Locrian mode if you want, but I find it easier to just think of D natural minor starting on its second scale degree. Notice the tritone between the E and B-flat, that’s what gives the chord its edge.
When we get to A7, we run into a problem: you can’t actually make that chord from D natural minor. The chord you get from the fifth scale degree of the D natural minor scale is Am7. It sounds okay, but it lacks drama and bite. There’s no leading tone, and no tritone.
We really need an A dominant 7th chord, with its tense and dramatic C-sharp. To make it, we’re going to need to use a different scale, namely, D harmonic minor:
Building a seventh chord from the fifth scale degree of D harmonic minor gives the A7 we were looking for. It’s got the leading tone, it’s got the tritone between C-sharp and G, and it really gives D minor a strong and unambiguous D minor-ness.
The V7 chord from harmonic minor has an additional interesting property. If you extend it by one more third, adding B-flat on top, you get A7(b9):
Now watch what happens when you leave the root off your A7(b9) chord. You get a C-sharp diminished seventh chord (or Edim7, or Gdim7, or Bbdim7–they’re all the same chord.) That C#dim7 chord works great as a substitute for A7 in D minor, with the added virtue of having that spooky “Dracula’s castle” vibe.
The fifth mode of D harmonic minor is called the A Phrygian dominant scale, otherwise known as Ahava Raba or Freygish mode. It’s the scale you use to play “Hava Nagilah” and many other songs of the Tribe. I tend to think of the A7 in D minor as coming from A Phrygian dominant, which is an especially useful framing for improvisation purposes.
So, to sum up: D natural minor is the basic scale for generating chords and melodies in D minor. However, when you need an A7 chord, you’ll need to switch over to D harmonic minor (or A Phrygian dominant, however you prefer to think about it.) Note that in more modern musics, you can further enrich D minor by bringing in D melodic minor, D Dorian mode, D Phrygian mode, and, of course, D blues. But that’s a subject for another post.