Progressions — the recurring sequences

Loop I–V–vi–IV — in G that's G–D–Em–C — and it sounds like finished music before any melody is added. This one sequence carries a substantial share of pop music.

A progression is a fixed sequence of those functional roles. It's written in Roman numerals because the numbers refer to the roles, not to specific chords. The same sequence is G–D–Em–C in G and C–G–Am–F in C: different chords, same effect. Learn a sequence once and it's available in every key.

Why do so few sequences carry so much music? Because they balance tension and resolution, and because the vi brings in a minor coloring. What matters most is the ending: I–V–vi–IV closes on the IV, which still strives back toward the resting point. The sequence never resolves completely and can therefore repeat indefinitely — that's the technical reason pop music is built in loops. A loop that turns needs a pattern to carry it, though: a beat, a when. That's where the next layer begins.