We begin with the Diatonic Triads of 'C' Major (Ionian)
'Aeolian' is the mode that uses the 'Root' (the 'Tonic') of the 6th 'degree.' This means that the VI Chord becomes a I Chord. But before we change the Chord Function(s) - let's look at the Diatonic Triads of Aeolian in their native habitat... their Relative Major Scale.
Now let's begin the work of understanding the Aeolian Mode as its own Key Center from the Tonic of 'A':
By Formula - the 'A' Major Scale is constructed:
When we remove the sharps (to get back to the notes of 'C' Major) -- we get the following formula:
This is the Formula for the Aeolian Mode: 1,2,b3,4,5,b6,b7 --or -- Major Scale (b3)(b6)(b7)... it's ok to memorize this because it will never change. Now, let's take a look at how the Diatonic Chord Functions of Aeolian have changed -- when it is considered to be its own Key Center [with 'Ami' functioning as the Tonic (the I Chord)]:
We might ask: Why bother learning two different chord functions for the same diatonic chords? The answer: because it changes the way they are heard (remember - this is all about training the ear). Even though the bVIma Chord of the Relative Minor Key Center is really just a IVma Chord in the 'Mother Scale' -- the root movement (in the bass) of a bVIma returning to its Tonic (Imi chord) sounds very different than the root movement of a IVma returning to its Tonic (Ima).
There is also something else to be mindful of at this point: any Minor Triad that we ever hear in our lives - will (most naturally) be heard as a VImi - IImi - or IIImi - in a Major Key Center - or - as a Imi - IVmi - or Vmi in a Minor Key Center. Likewise - any Major Triad that we ever hear in our lives - will (most naturally) be heard as a Ima - IVma - or Vma in a Major Key Center - or - as a bIIIma - bVIma - or bVIIma in a Minor Key Center.
This discussion has been about 'Relative Minor' (often referred to as 'Natural Minor'). Because the chord types have not changed within the diatonic sequence - it can be said that this discussion has been limited to 'Aeolian' - and could also be referred to as a discussion of Modal Minor or Diatonic Minor. In Part II of this series - we will explore 'Parallel Minor' in a way that is also 'restrictive' to diatonic ideas -- this restriction is for the purpose of training the ear for things to come.