Determining chords in a key

Question

I've been dabbling in music for years but don't have a lot of theory background. I'm trying to confirm a pattern I've seen in YouTube videos to see if I can reason out all the diatonic chords in a given key or mode.

For the sake of simplicity, I'm going to use the example of D Dorian so that I don't have to type a bunch of sharps and flats. If I want to know all the diatonic chords in D Dorian, is this a reasonable approach?

• Determine the D Dorian scale. That should be: D E F G A B C
• Build a list of chords based on the 1st, 3rd, and 5th for each of the chord tones. For D Dorian, this should result in:
• D F A (Dmin)
• E G B (Emin)
• F A C (Fmaj)
• G B D (Gmaj)
• A C E (Amin)
• B D F (Bdim)
• C E G (Cmaj)

If that is all correct, would it also be correct to determine all the diatonic 7th chords the same way? In other words, starting with the same D Dorian scale as before, build a list of seventh chords consisting of the 1st, 3rd, 5th and 7th notes, starting from each chord tone:

• D F A C (Dmin7)
• E G B D (Emin7)
• F A C E (Fmaj7)
• G B D F (G7)
• A C E G (Amin7)
• B D F A (Bdim7)
• C E G B (Cmaj7)

If everything so far is right, am I right in assuming I can build all the 9th, 11th, and 13th chords in exactly the same way by just adding the 9th, 11th, and 13th notes to the chords? (I'm not going to attempt that here because it's likely going to raise more questions as to what to name each chord; I'll save that for a followup question.)

If this is all right, then I'm kicking myself that I didn't see this years ago! This opens the door to putting the appropriate chords against a melody, which has always seemed very mysterious to me since I was never shown how to figure out the right chords to use.

I realize this is only a formula for figuring out diatonic chords and doesn't even touch the wondrous things you can probably do with chromaticism but I figure I should walk before I run :-)

Answer 1

You are correct. To build diatonic chords, one just sticks to the scale, using every other note. So another way to express your list of Dorian triads would be

1-3-5
2-4-6
3-5-7
4-6-1
5-7-2
6-1-3
7-2-4

And that works for every diatonic scale/mode (including seventh and extended chords), with only the chord qualities changing from mode to mode.

The primary exception is in minor, when convention states that the major seventh degree is used rather than the minor seventh. If you're writing modal (aeolian) music, you'd use the minor seventh; if you're writing tonal (minor) music, you'd usually raise the seventh.

Also, one chord is mislabeled: Bdim7 should be Bhalf-dim or Bmin7b5.

Answer 2

Maybe a simpler way to understand what's going on would be to realise that D Dorian is the Dorian mode from the parent key C major. And because of this, every chord you quote is exactly the same as those spawned from key C major.

Diatonically the notes must be the same set, so nothing will change.

You have indeed found one of the Holy Grails of music! In any key, there are three major triads - I, IV and V, and three minor triads - vi, ii and iii. The minor triads equate to i, iv and v of the relative minor key. The remaining triad is vii, a lesser used harmony, although the notes involved blend with V to produce V7 - the dominant seventh chord for that I key.

Answer 3

It's very simple: there exists an idea that when chords are built from nothing but the notes of the scale in which the melody of the music is based, they blend with the music. That scale is the primary source for chord tones. You seem to have hit upon this idea and are excited.

However, it is an oversimplification to the point of almost being false. Or at least far from being the only truth. Likely on a daily basis, you are exposed to all sorts of music which uses harmonies that are off the reference scale. It is not entirely obvious for that reason and probably why you didn't latch onto it sooner.

Here is why:

1. Music that is composed in reference to a scale does not use all the notes of the scale all the time, in every measure or passage. Any notes which are missing create a possible ambiguity. If we look at the notes which are present in that chunk of music that we are harmonizing, we can imagine them as belonging to multiple different scales, and therefore even if we stick with the "chords come from parent scale" concept, we can bring in notes from a choice of different scales, not necessarily the single reference scale for the entire piece, and do so to a useful musical result.

2. Not all combinations of notes from the same scale blend equally well. Some chord made entirely of the scale notes can be quite dissonant against the melodic passage from the same scale, even in comparison to a different chord which uses off-scale notes. This point is not entirely separated from the first point. (Note that "dissonant" doesn't mean "bad". We are talking about smoothly blending or not, without passing judgment. If you listen to, all day long, to nothing but music in which everything blends smoothly, you will probably get bored.)

The parent scale concept is useful, though. E.g. George Russel's Lydian Chromatic Concept is strongly rooted, no pun intended, in the doctrine that chords are derived from parent scales.