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In SATB, the chord iib in D-flat major, the chord is realised as:
G-flat, E-flat, B-flat, (G-flat is doubled).

Why isn't the E-flat doubled, it being the root of the chord?

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Why is G♭ doubled? Because whoever wrote the music felt that sounded good.

Why was that a valid choice? Because there's no rule against doubling the 3rd of a minor chord. The 3rd of a MAJOR chord is a much more active note, often with a leading-note function, wanting to resolve upwards. Doubling it would either give parallel octaves, or make one instance resolve awkwardly. The 3rd of a minor chord doesn't have this issue.

Also, ii contains the subdominant note of the key. Tonic, subdominant and dominant are 'keystone' notes in the key. Traditional harmony classes every chord as having either tonic, subdominant or dominant function. They can be doubled, even take preference for doubling, when they occur in any diatonic chord.

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  • Often the best note of a chord to double is either the 1st, 4th, or 5th degree of the scale. This is because these notes emphasise the harmonic function of the chord: tonic function, subdominant function, or dominant function, respectively.
    The supertonic chord (chord ii) usually has a subdominant function, so emphasising the 4th degree of the scale is good.

  • Doubling the third of a minor chord is fine.

  • It's also the bass note of the chord (as it's in first inversion).

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A note for others: the b in iib means "inversion b" or first inversion, or ii6 using figured bass numerals.

This can be discussed without any reference to specific keys or tones, and I think it's easier to do it that way, it eliminates unnecessary detail.

Why would the third of the chord be doubled in a ii6 chord?

The principle I learned from Walter Piston's Harmony is IMO the most logical and easiest to remember:

  • in root position double the root
  • in first inversion double a tonal degree those tonal degrees being the tonic, subdominant, or dominant (the modal degrees are mediant and submediant, and the supertonic is sort of a tonal degree and the leading tone is it's own specific category)

There are some other rules of thumb regarding doubling tones, but we don't need them for this case.

In a ii chord the tones/degrees are:

  • root is the ^2/supertonic/solfege RE
  • third is the ^4/subdominant/solfege FA
  • fifth is the ^6/submediant/solfege LA

In ii third is the only tonal degree - the subdominant - so when inverted to ii6 double that one, the chord's third.

If you follow the same process for another chord, for example the other subdominant chord, the IV, you will get different results. With an inverted IV6 you would double either the root or the fifth using the Piston rule. So, again, the particular chord member isn't important, it's the place of the tones within the tonality (which scale degree are being used) that is important.

And the important rationale for that rule of thumb is: this is tonal music, emphasize the tones which make clear the tonality. The tonal degrees do that whereas the modal degree define the mode.


Just a note about doubling and minor chords. Some times you see a rule about doubling in minor chords, usually list with a long list of other caveats. In those cases the detail usually missing is chord/scale degree position in the tonality. When minor chords are considered as ii, iii, and vi in the major mode their respective scale degrees are subdominant, dominant, and tonic. That brings us back to the simple rule from Piston: just double tonal degrees. If, however, the minor chord was in the minor mode, like a minor i, then the Piston rule would say double the tonic or dominant, but not the mediant.

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