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Not knowing the historical reasons, I can at least think of structural reasons for having

  • a diatonic scale,
  • equal temperament or other circulating temperaments, and
  • a distinction in major/minor in mood and harmony.

But we have seven different modes in a diatonic scale, of which we can characterize three as major (Lydian, Ionian, Mixolydian) and three as minor (Aeolian, Dorian, Phrygian) based on the triad above the root.

Why is it that of these three major and three minor modes, Ionian has become prevalent and known as the Major Scale and Aeolian has become prevalent and known as the Minor Scale? Why do Ionian and Aeolian get to be the frontrunners here?

And while I suspect the reasons are purely historical, can anyone nonetheless think of structural reasons? Why should Aeolian be the prototypical minor mode and Ionian the prototypical major mode?

By the way, I found the video Why C?: The Convoluted History of Note Names by Music Corner, see here. He claims that tonality “basically appears out of nowhere” at 4:19. I don’t buy that. Do we really no nothing of the actual historic origins of tonality and the Major and Minor Scale?

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    Ideas for structural reasons: Both Ionian and Aeolian are, by brightness, in the middle of the major modes and the minor modes respectively. Mixolydian has no leading note, Lydian has no perfect fourth. But that still leaves Dorian and Phrygian as possible candidates for the Minor Scale.
    – k.stm
    Jan 23 at 9:49
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    I'm not sure that the natural minor is the prevalent minor scale in terms of actual usage..?
    – topo morto
    Jan 23 at 10:30
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    @k.stm Where is it taught as the minor scale? Scales are only helper reference grids for reasoning about harmony: pitches and their relationships. If you look at actual music, you'll notice that any one single scale is rarely enough for the entire piece and its arrangement. The natural minor scale is what you get as your default reference pitch grid from the key signature. If you want to implicate a real dominant chord going to a minor tonic, or a secondary dominant, or diminished sevenths or something, you'll need to deviate from the default reference scale. Jan 23 at 10:58
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    In other words, I think it's fair to say that the natural minor scale is taught as 'prototypical', but it's perhaps not necessarily the case that it's prevalent in terms of usage (or necessarily taught as such).
    – topo morto
    Jan 23 at 11:05
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    BTW a couple of other possibly interesting questions: music.stackexchange.com/questions/19033/… and music.stackexchange.com/questions/77060/…
    – topo morto
    Jan 23 at 11:08
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As you point out, of the seven modes of the major scale, the Ionian (major) and Aeolian (natural minor) are 'preferred', or perhaps used more often. Also, the labeling system seems to accord special paradigmatic value to these two modes.

Now one should note right of the bat that there is plenty of music (popular and otherwise) that uses melodies and chord sequences that are derived from the other modes, especially the Dorian, Mixolydian, and Lydian mode.

To come to your question: is there any structural difference between the Ionian and Aeolian mode, and the other five modes of the same system? Here is one structural difference that is often overlooked, which is related to the tonic or 'home chord' of the section of music that is composed in major or natural minor setting:

In the context of tertiary harmony, which means harmony in which the chords are formed by stacking thirds from the scale, we should expect each of the seven modes to come with a unique set of chords (major/minor, but also seventh chords and up) around a given tonic chord.

If we take C major as an example, the home chord is the C major chord (C, E, G). For the piece to be meaningfully "in C major", we have to use the notes of C major but also use the C major chord as a home chord.

If we now switch to the relative minor scale of C major, we get A natural minor. Again, this means that we have to use the notes of the A minor scale as well as the A minor chord (A, C, E) as a home chord.

For each of the five other modes, we get an analogous story. Now what is special about using A minor or C major as a home chord in this context? The short answer is that these two chords are the only two triads that exist within this system which contain both the notes C and E. The reason that this is of interest is that the C major scale (or any of its equivalent modes) contains exactly one tritone interval, between B and F. This interval is often used to generate tension. For that tension to resolve, both notes of the tritone need to move to adjacent pitches of the system, to a consonant interval. Using all the pitches of C major, the notes C and E are simply by far the most pleasant way to resolve the tritone between B and F.

This explains why A natural minor (with A minor as a home chord) and C major (with C major as a home chord) are special. These are the only modes/key centers which contain the interval (C-E) which is the optimal way to resolve from the tritone. So whenever tritone resolution is involved to go on musical journeys (tension-release), then the Ionian and Aeolian modes are good, because their tonic chords are those which feel resolved when coming from a tritone.

Incidentally, in the period that these labels were invented, tritone resolution within this type of system was the preferred approach to tension and release in music. So the labeling system reflects the fact that only these modes can be 'home', if 'home' means "a chord which has an interval to which the tritone resolves."

This is of course the beginning of the story. It also contains the beginnings of an explanation why the harmonic and melodic minor scales were invented. I hope this helps!

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  • Do you have any evidence that the Aeolian is particularly preferred, or is it your personal impression? I ask because we had a question recently: Which form of minor scales is the most common to use in songs in minor keys - and it was closed as opinion-based...
    – topo morto
    Jan 23 at 13:22
  • the OP only asked about the modes of the major scale, so that excludes the harmonic and melodic minor. I would say that at least the natural minor, Dorian, Melodic and Harmonic minor are all used frequently (and also mixed with each other), although I wouldn't want to have to bet on the distribution between them. I agree with your point that it's not useful to designate one of them as THE minor scale. Jan 23 at 13:26

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