It may seem that these posts are back to front compared to many books. There is a reason for that. I’m not explaining the mechanics of playing an instrument. Books that do this usually start with a few notes or chords to get you going. These posts are more aimed at people who can already play but want to learn more about how music is constructed.
This means I can introduce the scale / major Key system first as I think chords then make more sense because you can see where they fit in.
It also means you’ve done all the hard work already so this is the easy part.
Furthermore, I can then introduce the structure of chords independently of Keys and you can go and work them all out for all keys if you like. This is why introducing intervals then the circle of fifths / fourths first really helps as it’s the foundation for working out any chord you want to.
A chord in essence is basically a group of notes played at the same time. The simplest types of chords are known as triads and contain 3 notes. A major triad contains the root, major third and perfect fifth; A minor triad contains the root, minor third and perfect fifth; A diminished triad contains the root, minor third and diminished fifth.
We can now work out each chord for each of the keys using these formulas and referring to the circle of fifths:
Minor – I, bIII, V
Diminished – I, bIII, bV