Aeolian mode harmony

Question

I have been having trouble writing progressions in other modes, and I think this question might get me one step closer to understanding the modes (in this case just Aeolian) from a different perspective.

The Aeolian mode is a diatonic scale also called the natural minor scale. On the white piano keys, it is the scale that starts with A. In other words, this scale contains all of the same notes as the C major scale--just in a different order. That being said, there is still something called Aeolian harmony, which has (apparently) a somber quality to it. Why is this? What restrictions are placed on the a minor scale that makes it feel "Aeolian" and not simply like the minor chords from the C major scale?

Can someone explain why the leading tone is avoided?

I understand modes insofar as they are formulas for scales. I never really understood their implication for harmony and chord progressions, though. Does all theory about functionality (aka tonic-dominant relationship etc) go out the window? Are there cadences in the Aeolian mode (is a PAC in Aeolian what we would usually call a deceptive cadence?) Such a confusing concept for me.

Thanks!

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Answer 1

The notes are in the same order as a C major scale, it's just that the start point is different.

The root note, or home, if you prefer, in C major (Ionian mode) is C. All the other notes bear some sort of relationship to that C, in particular, rather than any other note. (Although every note has some relationship with every other). And one main difference that that has compared with A Aeolian is that there is a leading note one semitone under that C. It's B, and has an effective pulling power towards that C.

The root in A Aeolian is A. But there is no leading note as such - the note below (G) is a full tone under, and that pulling effect just isn't there. The leading tone isn't avoided - it's just not there! Of course it can be there, as G♯, but then we're not in Aeolian, we're in harmonic or melodic minor - different beasts. So, without it, Aeolian is , well, Aeolian. That's one defining feature it possesses.

From a different angle, the dominant chord harmony in key C has GBDF, with a tritone, whereas A Aeolian has EGBD, with no tritone, which means there isn't such a powerful pull back 'home'.

Answer 2

What makes the modes differ is not the order of notes, but the tonal center. It is a note that feels like home, that resolves the tensions. In C ionian it is C, while in A aeolian it is A.

In fact you can often take a melody, and depending on what chords you play along, it will feel like belonging to a different mode.

Besides the tonal center in order to produce aeolian harmony sound one may want to play the characteristic notes of the aeolian mode: minor 6th and natural 2nd. Without them, one cannot distinguish aeolian from dorian or phrygian.

Lack of the leading tone implies that there is no dominant chord. If you strictly follow aeolian mode, you are more in modal harmony which doesn't use harmonic functions as tonal harmony does.