Harmony

Chords covers how a chord is built. Harmony is what happens when chords are put in order.

The central idea is that in a key, chords are not interchangeable. They have tendencies. Some feel like arrival, some feel like preparation, and one in particular feels like it is pulling you home. Once you can hear that, chord progressions stop being lists to memorise.

The four ideas here

Which chords belong. Build a triad on each note of a scale using only notes from that scale and you get seven chords whose qualities follow a fixed pattern. These are the diatonic chords, and they are why progressions transpose so cleanly.

How to name them portably. Roman numerals describe a chord by its position in the key rather than its letter, so I–V–vi–IV means the same shape in every key.

What they tend to do. Harmonic function groups the seven chords into three jobs — tonic, predominant and dominant — and most progressions are a circuit through those jobs.

How phrases end. A cadence is the chord pair that closes a phrase, and the choice of pair decides whether the ending sounds final, questioning or deliberately avoided.

And then

Chord progressions puts the four together and works through the handful of patterns that account for an enormous amount of real music. Modulation covers what happens when a piece changes key mid-way.

What to read first

You need triads and key signatures before any of this will land. Intervals underneath both.