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How can we classify chord progressions?

In the context of tonal music, not all chords are created equal. In fact, there are three distinct families of diatonic chords (chords in the scale): Tonic, Subdominant, and Dominant.

  • Tonic – This family of chords consist of I, vi, and iii. These chords are considered “stable” and are often used as starting and ending points in a phrase.
  • Subdominant – This family of chords consist of ii, IV, and sometimes vi (depending on context). Subdominant chords serve as a stepping stone or a bridge between the Tonic function chords and the Dominant function chords.
  • Dominant – This family of chords consist of V and viio. Dominant function chords are considered “unstable” and they push us towards Tonic function chords.

After reading the above, you might see a pattern emerging:

Tonic – Subdominant – Dominant – Tonic.

This, in a nutshell, is functional harmony. Chord progressions built with these relationships in mind will, more often than not, sound complete. Keeping these relationships in mind will help the composer understand how chords can express emotion and tell stories, it will help the arranger find interesting ways to re-harmonize a melody, and will allow performers to make informed decisions in their approach to phrasing.

Ok, enough with waxing poetic, let’s talk about some chord progressions!

Up first is a classic progression from many popular genres, the I-vi-IV-V.

This progression is sometimes referred to as the “Fifties progression” and can be heard in such songs as Earth Angel, Stand by Me, and even in recent pop music like Perfect by Ed Sheeran. Notice that we start with two Tonic function chords: I and vi, followed by the Subdominant (IV), and ending with a Dominant (V). Since this is a repeating progression, it’s fitting to have a Dominant function final chord so we feel compelled to repeat back to the opening Tonic function chord.

This progression actually predates Rameau’s work by at least 20 years. If you’ve ever attended a wedding odds are you’ve heard it before.

This progression is twice the length of the previous example and it has a few interesting moments we need to discuss.

  • The first three chords (I-V-vi) move quickly from a Tonic function, to a Dominant function, back to a Tonic function. While this is common enough and still follows the rules of functional harmony, there’s a special name for what’s happening here that is useful to know: Deceptive Motion. This can be defined as using a Tonic function chord other than I as a point of resolution after a Dominant function.
  • Overall this progression has a way of meandering through the functions. In fact it’s not until the last three chords (I-IV-V) that we see a definitively clear Tonic – Subdominant – Dominant progression. This is not uncommon in classical and jazz harmony, however, in popular music it’s rare to find a progression of this length. What’s important to keep in mind at this point is harmonic functions can be applied at a micro and macro level. Here’s an example of each:

Micro Function

Things can get confusing if we were to consider the function of each individual chord without looking at the greater context.

Macro Function

In some passages it makes more sense to look at groups of chords as serving the same function. The first four chords are mostly tonic functioning chords with a dominant thrown in for “flavor”. The IV-I-IV is mostly subdominant with the tonic I chord used to lengthen the section. The Dominant chord stands alone in this case, but in the music of the romantics and post-romantics the Dominant function becomes one of the primary foci for extension.