A chord progression is a sequence of chords, written with roman numerals so it works in every key. I-V-vi-IV alone underlies thousands of songs.
Numerals name each chord’s scale degree: in C major, I is C, IV is F, V is G, vi is A minor. Hear the numerals and you can follow a song’s harmony in real time, in any key.
Cadences are how phrases end: V to I closes firmly (perfect), IV to I softens (plagal), a stop on V leaves the door open (imperfect), and V to vi is the deceptive swerve.
Common questions
What is the most common chord progression?
I-V-vi-IV and its rotation vi-IV-I-V: the four-chord loop behind an enormous share of popular music.
What is a deceptive cadence?
A phrase that sets up the usual V-to-I ending but lands on vi instead, withholding the expected resolution.