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The web site http://www.tonalcentre.org states that there are only two "tonally effective" modes of the diatonic scale (viz., the Ionian and Aeolian modes) [http://www.tonalcentre.org/Scales.html] and that there is only one "tonally effective" mode of the harmonic minor scale (viz., its first mode, i.e., the harmonic minor scale itself) [http://www.tonalcentre.org/Harmonicmi.html]. It indicates that, to be tonally effective, a mode must: "support a tonic triad. That is, it must have a chord which serves as a chord of rest and completion, as the tonal centre against which all the other triads are measured and towards which all gravitate."

I find this definition to be ambiguous, and I have not found any other definitions. I wonder, for example, why D Dorian mode does not "support" Dmin as its tonic triad?

Could somebody please clarify the definition of "tonally effective"? Thanks.

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    You better ask the author of that website, because nobody else uses that term. – Matt L. Dec 13 '18 at 7:31
  • Many pieces which are in Dorian or Mixolydian would beg to differ. 'Just because it's on the 'net doesn't mean it's true' - or any more than someone's opinion. – Tim Dec 13 '18 at 8:43
  • @Tim: Granted, but this web site concerns quite technical issues and is well written and thorough. It may contain mistakes, but I doubt it is promulgating music theory conspiracies. – Gidfiddle Dec 13 '18 at 17:06
  • @Matt L.: You're right, of course. I just sent e-mail to the author. – Gidfiddle Dec 13 '18 at 17:10
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I think he's got the right answer but the wrong reason. There's no problem hearing a tonic as 'home' in any of the modes. But tonal harmony is all about dominant-tonic relationships, about there being a chord that has a strong tendency TOWARDS the tonic. You need a major dominant chord, containing the leading note a semitone below the tonic, and all the better if it has a minor 7th to complete the tritone.

This only happens in Ionian and Aeolian (harmonic) modes. Which is why those two have been promoted from 'mode' to 'key' status, and are the basis of Common Practice music.

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  • It does seem reasonable to require a dominant-tonic relationship in defining "tonally effective" modes. Indeed, for the C Ionian mode, in the authentic cadence (V I), the V chord (the G major triad) contains the leading tone (B) for the tonic C, whereas for the D Dorian mode, the V/ii = vi is the A minor triad, which does not contain the leading tone C# for the tonic D. However, the same can be said for the A Aeolian mode: the V/vi = iii is the E minor triad, which likewise omits the leading tone G# for the tonic A. So again: Why is A Aeolian "tonally effective" whereas D Dorian is not? – Gidfiddle Dec 13 '18 at 15:45
  • So what's wrong with the Dorian harmonic mode? Thanks to being harmonic and therefore having a leading tone a semitone below the tonic, it has a major dominant note and, thanks to having a conventional subdominant note, a genuine dominant 7th for the dominant. – Dekkadeci Dec 13 '18 at 16:05
  • How did Aeolian morph into harmonic minor? If that's true (and it is) that a leading note is needed, then the Lydian mode is also in the frame, as far as the author should be concerned.And Aeolian shouldn't be there. – Tim Dec 13 '18 at 17:15
  • @Dekkadeci - please help me with this 'Dorian harmonic mode. A mode to me is a bunch of notes diatonic to a key, but rooted on another note. Changing a note seems quite convenient, but wouldn't work for all modes. Lydian with a raised seventh note, anyone? – Tim Dec 13 '18 at 17:19
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    @Tim - I named the "Dorian harmonic mode" in analogy with this answer's "Aeolian (harmonic) mode"--like the harmonic minor is the Aeolian mode with a raised seventh, the Dorian harmonic mode would be the Dorian mode with a raised seventh. I agree that the analogous Lydian (and Mixolydian) harmonic modes would be silly. – Dekkadeci Dec 13 '18 at 17:38

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