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Do triads from all forms of minor count as "diatonic triads"?

Triads from A Aeolian / Nat Minor

am C dm em F G

Triads from A Harmonic Minor

am C+ dm E F g#˚

Triads from A Melodic Minor

am bm C+ D E f#˚ g#˚

It seems like I mostly see this:

am C dm (E or em) F (G or g#˚)

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Theorists differ on the precise usage of "diatonic" when it comes to minor. There isn't any substantial disagreement on the behavior of minor-key common practice music, just in the best use of terminology. Under the most expansive interpretation of the word, any notes of the minor scale, including both versions of the sixth and seventh scale degrees, are diatonic, and thus, at least technically, any chords using those notes would be diatonic. This is the same as saying that all the chords in the natural, harmonic and melodic minor scales are part of the key—however, I would prefer to step back from the idea that those scales are specific entities in common-practice music. Until the 20th and 21st centuries there was no such thing as a piece in "A harmonic minor" or "A melodic minor"; there was just "A minor" which has variable sixth and seventh scale degrees depending on musical context.

The most restricted use of "diatonic" holds that only the most common progressive (not sequential or passing) harmonies should fall under the term. By far, the most common functional harmonies in a minor are:

amin bdim C dmin E F g#dim

or:

i ii° III iv V VI vii° 

That is to say, the chords of the "natural" minor scale except with a raised leading tone for V (E) and vii (G#).

I don't care much about how we use the diatonic term, as long as we're clear that these are the most important harmonies for functional harmony in minor.

The only other harmonies that the most expansive definition of diatonic would add to the list all involve the sixth or seventh scale degrees of course, and their prominence varies substantially. The V and vii without the raised leading tone aren't that uncommon, but they are vastly more significant in sequences (e.g. i – iv – bVII - III …) and in passing harmonies (e.g. i – v6 – VI). The sequence example is the beginning of a minor-key circle-of-fifths sequence with the major VII built on the unraised seventh scale degree, and the passing example is a minor v in first inversion (unraised leading tone) to pass between i and VI. Meanwhile, unlike V and vii, III almost always has the unraised leading tone, and is quite rare as an augmented harmony III+.

All of the standard harmonies that include the sixth scale degree use the unraised version, and the versions with raised 6 are relatively rare. Probably the most common is IV, which is sometimes necessary to create smoother voice leading to V (in other words, to eliminate the augmented second between the lowered sixth scale degree of iv and the raised seventh scale degree of V). It's less common to do this with ii (turning a diminished triad into a minor triad), but not unheard of. Raising the sixth scale degree of a VI chord in minor turns it into a diminished harmony (often written in Roman numerals as #vi°) that is used as a passing chord leading up to vii° or V6. Again, that is used to eliminate what would otherwise be an augmented second in the bass line.

So, yes, I think your final list is a good list of the standard diatonic harmonies in a minor as long as you get rid of e minor and G major. Those two harmonies mostly happen in sequential or voice-leading circumstances. I think it's probably most useful however to acknowledge that the less common versions of all of the sixth- and seventh-scale-degree-having harmonies aren't chromatic (and hence are indeed diatonic), they're just less common and/or used for other contexts.

  • There are quite a few pieces in A minor which use Em and/or G for bars in their own right, which I think means they're not being used in a voice-leading manner. The G seems far more common than Em in my experience. Example - Spanish sequence. – Tim Jun 10 '17 at 8:09
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    @Tim I'm not familiar with the Spanish sequence, but if it is indeed a sequence, then that's what I meant when I said v and bVII are more common in sequences. If by "in their own right" you mean Emin functioning as a dominant chord just as E does, or G functioning as a dominant chord just like G#° does, all I can say is that, although certainly exceptions can be found, these would be highly unusual in a common-practice context. Of course, toward the end of the 19-century and throughout the 20th and 21st, both become quite common, and, in some genres, surpass E and G#° in prominence. – Pat Muchmore Jun 10 '17 at 8:20
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    I think the "Spanish sequence" is formally known as the Andalusian Candence if anyone is interested. – syntonicC Jun 10 '17 at 21:19
  • Ah, if that's what we're talking about, then it's definitely a passing function. The major VII chord serves as a connection between the i and the VI. A slightly variation on the minor v6 example I discussed in my answer. – Pat Muchmore Jun 10 '17 at 22:07
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    @RailroadHill No, the augmented III+ is nowhere near as common as major III in common practice harmony. You’re right that it can happen, which is why I said it’s rare rather than nonexistent in my answer. However, Chopin’s use of it is evidence of how much more complex his Romantic-era harmonies are, not proof that it’s standard Classical harmony. – Pat Muchmore Jan 16 at 0:10
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Diatonic relates to major and minor keys - as opposed to chromatic. So, yes, any triads made up from notes in all of the scales (and their modes!) you quote are diatonic. Note that a triad needs root, third and fifth, so a sus chord isn't a triad; and a triad can be non-diatonic, as in diminished and augmented, still with R, 3 and 5, but not all diatonic notes.

Those from the natural minor are exactly those from the mother key - the relative major. With the altered notes as in the last two found in melodic and harmonic minors, there are more triads available, all of which can and do feature in minor pieces - and those which have modulated there.

EDIT: Trying to clarify - 'diatonic' in a situation means 'belonging to the set of notes in question', so here we're talking about the standard major and minor which are used extensively in Western music.

  • Good note on the sus chord not being a triad. Never thought of that before. Minor is very expansive having 3 different forms. – Kolob Canyon Jun 10 '17 at 8:34
  • No! A triad comprises 3 notes. They do not have to be "root", "third", or "fifth". In music without a tonal center (serialism or set theory), there are no roots, thirds, or fifths (unless manufactured by the composer). Also, to clarify, "diatonic" is neither major nor minor, but describes notes normal to the parent scale / key. If you create a synthetic scale that doesn't fit into any key, the chords you create from that scale, as long as they don't deviate from the scale, are diatonic to that scale. – jjmusicnotes Jun 10 '17 at 13:06
  • @jjmusicnotes - according to most of what I've read, triads, in general diatonic Western music, are made up from notes in thirds, comprising root 3 and 5. I wasn't addressing any other sort of music. Diatonic, again as I have studied, refers to notes from major and minor scales and keys, and I felt that the OP was only asking about that sort of music, not any synthesised scales, although I take your point. Here we talk about diatonic in the major/minor form. Do you agree that a sus 2, e.g. C F G is not a triad? – Tim Jun 10 '17 at 13:32
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    @jjmusicnotes There may be a difference in how this term is used in different countries—that's definitely the case with terms like "12-tone" and "serialism"—but at least in the American tradition I learned, "triad" does indeed mean three-note stacks of major and/or minor 3rds (including, of course, 6/3 and 6/4 inversions too). The term "trichord" is used for the more general case of any three-note collection. In that sense, C–E–G is both a triad and a trichord, while C–D#–E is only a trichord and not a triad. Again, though, I think there's a lot of variability on this. – Pat Muchmore Jun 10 '17 at 13:44
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    @PatMuchmore Absolutely agreed here. I've always used triad / trichord (for better or worse) interchangeably in non-functional contexts, but I do always use "triads" in functionally-harmonic contexts, or perhaps "traditional" is the better word here? I fully realize successions of triads may be entirely non-functional harmonically speaking. In my travels and career, I've noticed, as you said, a lot of variability in how people speak about and notate musical concepts. – jjmusicnotes Jun 10 '17 at 15:43

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