This part is on The Minor Key, and chord formation from the minor key. The below follows on very directly (and refers a lot to) from part 5 : The Major Key and chord formation, so make sure you get to grips with that first.
So, as discussed in previous posts, the minor scale formula is : T S T T S T T
This is the same formula as if you started from the 6th step of the major scale. (This links into the concept of Relative Major/Minors). Chord formation works in exactly the same way as a Major Key, ie. root, miss one, third, miss one, fifth.
So the bottom line is, given that the minor scale is the same formula as the major scale from the 6th note, we can assert that the formula of chord types in a major key is the same as the major scale from the 6th chord. To clarify what this means, let’s look at it in detail.
Major Key Chords – Imaj IImin IIImin IVmaj Vmaj VImin VIIdim
(The VImin is highlighted as this is where we start our minor key formula from. Thus it is as below)
Minor key chords – Imin IIdim IIImaj IVmin Vmin VImaj VIImaj
Again, this formula is all-encompassing, ie. take a minor scale, the chord built from the first note is minor, from the second is diminished, from the third is major and so on, without fail, exactly replicating the major scale from note 6.
You should learn and know these two formulas, but also understand their relationship to each other to support the knowledge.