Chord progressions are the basis of harmony in songs and there are many common or standard progressions that are useful to know.

(a chord progression is simply a sequence of chords: one chord after another.)

Chord progressions are often thought about in terms of the chord roles in a particular key, rather than the exact chords played.

For example, we talk about a “1 4 5” progression (or “I IV V“) rather than a “C F G” progression, because playing the chords with these scale degrees will produce the same characteristic sound in every key.

There are music theory rules for what chords can or should follow each other, and these are helpful to know because: