Harmony
Harmony is the study of what chords do to each other. Not which ones are allowed, but where each one pulls and what happens when you refuse to follow. Every page here plays the idea with and without the thing it is about.
Function before names.
Read Harmonic Function first. Once you hear that the same chord means different things in different keys, the rest of this pillar stops being vocabulary and starts being useful.
Jump straight in
The foundation
how chords get their meaning
Going outside the key
colour, borrowing and travel
The written tradition
where the rules came from
Keep going
The other sections
Everything on this site sits in one of these, in the order they are meant to be learned.
The building blocks
How music works