COMPLETE INTRODUCTION TO JAZZ
As music has evolved from classical, through to romantic, through to 20th century music sales (in particular jazz) – composers have gone from playing simple 3 note chords (root 3rd 5th), and gradually ventured out into the extended harmony notes – adding 7ths, 9ths, 11ths, and 13ths.
Chords have always been built of 3rds. IIn search of new sounds, composers of the romantic period started venturing out beyond the 5th. Why not try building the stack of 3rds higher, beyond the 5th? First they started playing 7th chords.
Then once people had got used to the sounds of 7ths in chords, they ventured further and started building 9th chords (composers like Ravell for example).
And then further, building 11th chords, and eventually 13th chords.
13 is the highest a chord can go, because if you build a 3rd above the 13th you find yourself back at the root of the chord. Otherwise we’d probably have carried on higher to 15ths, 17ths, etc.
So it’s not as though one day composers started using all sorts of exotic extended chords with b9’s, #11s and b13’s. Jazz harmony has been a gradual evolution over several decades.
Jazz chords are known for their complex sounding labels, but ultimately, these complex sounding labels are just names for these different stacks of 3rds. So there are many possible combinations of major 3rds and minor 3rds. You could build major 3rd – minor 3rd – major 3rd (C E G B) which is called ‘C major 7’. Or you could build minor 3rd – major 3rd – minor 3rd (C Eb G Bb) which is called ‘C minor 7’. So that’s all these chord symbols mean – they tell the player which stack of maj / min 3rds the chord is built of.