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I learned from a website https://www.mymusictheory.com/for-students/grade-6/170-a6-harmonizing-a-melody-i saying Chord III is rarely used. But, does the well-known Canon in D use chord III as the progression is D → A → Bm → F#m → G → D → G → A?

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    In key D, F#m is labelled iii, rather than III - which is better labelled V/vi.
    – Tim
    Mar 3, 2023 at 9:14
  • @Tim Isn't V/vi something slightly different? In this case, wouldn't it need an E? (I'm no expert, but it doesn't sound like a type of A chord to me, and I can't see what function an A chord would fulfil there.)
    – gidds
    Mar 3, 2023 at 12:06
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    @gidds - V/vi key D would be the dominant of Bm, hence F#(maj). V by itself would be, as you say, chord A, but that's not what's quoted.
    – Tim
    Mar 3, 2023 at 12:11
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    music.stackexchange.com/questions/127557/… here is a similar question that discusses the scarcity of iii chords.
    – nuggethead
    Mar 3, 2023 at 13:25
  • "Rarely" is not the same as "never". Question closed. Next please. Mar 5, 2023 at 21:34

3 Answers 3

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Yes, it does. This sequence is one of the places where the III chord appears. Another place one might expect it is in consecutive first inversion chords.

For another example (but a debated one), see iii9 - I progression in Bach?

The website reads:

Chord III is rarely [emphasis original] used. It is possible (but not recommended) to use it in a major key (where it is a minor chord, e.g. E minor in the key of C major). It is NOT possible to use an augmented III in a minor key, (e.g. C-E-G# in the key of A minor), but sometimes a major chord III is possible (because it’s the relative major chord).

In the context of explaining how to do an exam harmonization, this comes across as overstated. It's true that III is less common than other chords, and it's uncommon in harmonization exercises except when the exercise is intended to demonstrate a specific situation such as a sequence.

Aside from sequences, III will most often be found as a precursor to VI and occasionally as a substitute for I or as leading to II or IV.

But in general, it's a fair practice in harmonization exercises to make III the last chord you try.

There are lots of questions on this site regarding the III chord in various contexts, which you can find using the search "iii chord is:question".

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    I know what's intended, but it's odd that they use the phrase "It is NOT possible..." as if your instrument would break trying to play i → III+. I suppose when I play Am followed by C+, I hear it as Am and E+... maybe it's not possible after all.
    – Theodore
    Mar 3, 2023 at 15:00
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Yes, there is a iii chord in that Pachelbel's Canon progression.

It is a harmonic sequence called "falling thirds". It takes the two chord pattern I V and then sequences it by diatonically transposing it down a third to vi iii.

Some explanation of the "rare" label applied to iii should be given.

It is important to distinguish between the tonal chord I IV V and the modal chords ii iii vi and understand them in terms of basic harmonic function.

  • I is the tonic.
  • IV is the subdominant and ii is usually described as having subdominant or pre-dominant function.
  • V is the dominant and viio is usually described as have dominant function.

The list above covers chord I ii IV V viio and leaves out only iii and vi.

iii and vi are the principle modal chords and provide the major/minor flavor of a key.

iii and vi don't fulfill any of the functions: tonic, subdominant, dominant. We could say that the use of iii and vi is for modal color rather than harmonic function. vi is fairly common because of its use in deceptive progressions like V vi.

That leaves iii the odd chord out. That makes it sort of "rare." A likely time to hear iii is in a harmonic sequence, like the falling thirds in Pachelbel's Canon.

Don't misunderstand the general description of "rare" to mean "never used" or something like that.

Another way to think of the "rarity" is via the fundamental harmonic progression of roots by descending fifths.

We start with the roots by fifth descent to the tonic ii V I and add to that the two alternative chords (ii or IV) (V or viio) I, which gives us 5 of the 7 diatonic triads.

Add one more descending fifth to that... vi (ii or IV) (V or viio) I and we could say that vi is fairly "remote" from I.

Add one more descending fifth... iii vi (ii or IV) (V or viio) I and we could say that iii is the most "remote" from I.

Consider "remote" as synonymous with "rare" and this give us a sense of why iii is "rare."

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  • You're the first I've heard describe I-IV-V as "tonal" chords and ii-iii-vi as "modal" chords. That's a really helpful distinction ... which I am now stealing.
    – Aaron
    Mar 3, 2023 at 18:26
  • "Don't misunderstand the general description of "rare" to mean "never used" or something like that." A good point. Teachers wouldn't call it "rare" if it were in fact never used, of course, but students somehow frequently confuse the two nonetheless.
    – phoog
    Mar 5, 2023 at 17:09
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Chord III is rarely used. But, does the well-known Canon in D use chord III as the progression is D → A → Bm → F#m → G → D → G → A?

Well, yes, but only for part of the piece. For much of the piece, the fourth chord in the progression isn't F♯m but rather D/F♯ (and in three instances it's arguably D7/F♯ (V[6/5]/IV) with a C natural passing tone from the D to the B of the next chord). You might look at this as additional evidence in favor of the proposition that the iii chord is rare.

Because of this, iii appears in the piece with a frequency of around 1/16, with the full table of frequencies being roughly (combining I and V7/IV, and ignoring for now the difference between ii6 and IV because I haven't counted them yet):

I:   5/16
iii: 1/16
IV:  1/4
V:   1/4
vi:  1/8

So even in this unrepresentative sample, the iii chord is less prevalent than one would at first expect.

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