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I suspect that it might have something to do with tonality and functional harmony. A diminished or augmented tonic chord, due to dissonance, would be less satisfying and conclusive as the final chord of the piece than a minor or major chord - therefore the fifth degree has to be perfect. A high seventh degree makes the resolution to the first degree more satisfying, which is why the seventh degree is often raised in minor.

However, those constraints still leave us with many scales other than natural major and harmonic and melodic minor. (For example, Lydian scale.) Is there something else, other than a consonant tonic chord and the high 7th degree, that makes the major scale and harmonic and melodic minor scales particularly suited for tonal music - and this something is lacking in other scales?

marked as duplicate by Caleb Hines, Carl Witthoft, Peter, Dom theory Jul 31 at 17:09

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    I disagree that it is a duplicate. The other question is mainly concerned with the history of the terminology. My question is mainly concerned with the reason why the major key and the minor key with alterations became the most ubiquitous in common practice music and what makes them more suitable for tonal music and functional harmony. – Liisi Jul 31 at 13:18
  • It's close enough to the answers there; in any case there are several other questions discussing the use or misuse of all the currently named scales and modes. – Carl Witthoft Jul 31 at 13:31
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Is there something else, other than a consonant tonic chord and the high 7th degree, that makes the major scale and harmonic and melodic minor scales particularly suited for tonal music - and this something is lacking in other scales?

One thing is that to establish the tonic, it's useful to have the perfect fourth and perfect fifth in the scale - these are its closest friends in terms of representing simple frequency ratios (P4 is 4:3, and P5 is 3:2). So I tend to think of the tonic, P4 and P5 as being the 'scaffolding' of tonal music. Of course these aren't lacking in all other scales - part of the genius of the diatonic scale is that 5 of its 7 modes have the P4 and P5 (although Lydian doesn't have the P4).

The major scale is a lot of fun in that you can base a very stable major chord on each of the tonic, P4, and P5. This gives the ability to establish the tonic strongly, but also to create a strong sense of motion to the 4 and 5. The strength of the V-I motion (due to the V chord containing the leading note you mention) is useful in styles of music that like to mess around with modulations, and establish new tonics.

Minor tonality is fun because: cheating! We've thrown both types of 6th and 7th in there. We also have the P4 and P5 for stability.

Just a couple of thoughts - hopefully we'll get some answers with more of the historical perspective too.

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It's more than just the scales (and their notes) that makes it so. It's the blending of those notes. As in harmony, particularly chords, mainly triads, (based on stacked thirds).

The blend of those notes, giving three major chords - I, IV and V - work best in the Ionian mode - aka major tonality. I, IV and V (and/or minors) don't work as convincingly to our Western ears in any other modes - except perhaps, as you mention, Aeolian, and even then, the leading note begs to be sharpened.

Even playing in say, Dorian or Mixolydian modes, one has to be careful not to get pulled into Ionian, which is, after all, the mother of them all.

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All modes have a tonic. Many have a dominant a perfect 5th higher. (In C major and C harmonic minor that's C and G.) But when you also have a leading note, a semitone below the tonic (B) and a note that forms a tritone interval with it (F) - well, like the man said, "Then you got Common Practice Harmony!"

It's a system very much concerned with one chord having a dominant - tonic relationship with the next. A flattened third doesn't upset it too much (though the 'tierce de Picardie' major tonic chord at the end of a minor key piece was frequently attractive to composers.) And the flattened 7th in the melodic minor scale is tolerated when it IS used melodically rather than harmonically.

Western folk song has always rather liked other modes, notably the Dorian. Maybe it flows more naturally when melody is paramount. But for functional harmony, we need that dominant 7th chord to be 'in mode'.

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    The dominant 7th chord can certainly enrich the harmony and make it more complex and interesting. But a triad built on the fifth degree of the scale can fulfill the dominant function. You can have obviously functional chord progressions built from triads only. Therefore, I am quite sceptical about the statement that the dominant 7th chord is an absolute requirement for functional harmony. – Liisi Jul 31 at 13:35
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    It's not an absolute requirement. But it's very useful. Which may be why there's a WHOLE LOT more major key than lydian mode Common Practice music. :-) – Laurence Payne Jul 31 at 14:02

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