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I was playing around on the piano, looking through some old grade 3 books, and came across the chord sequence of Ab, Bb, C.

It sounds quite heroic, almost John Williams-like, something that would fit very well into film music, and yet I can't think what key these chords fit in to.

If they don't fit into a key, why do they, theoretically, seem to work so well?

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4 Answers 4

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This is a very common progression that is in fact equated with heroism and the "epic" sound we hear in so many soundtracks.

Assuming C is tonic:

In terms of Roman numerals, this is a VI--VII--I progression.

The chords you listed are

  • Ab C Eb
  • Bb D F
  • C E G

Which gives a scale collection of C D Eb E F G Ab Bb C, which is no scale collection that I know of, so there's going to be some aspect of chromaticism involved. This can happen in one of two ways, both under the umbrella term of "mode mixture" or "borrowed chords":

  1. The piece is in C major (hence the E in the final chord), but "borrows" pitches/chords from the mode of C minor (hence the term "mode mixture"), which is where we get the Ab and Bb chords. If you're in C major, you might want to clarify the Roman numerals to show these alterations: bVI--bVII--I.
  2. The piece is in C minor, so now the Ab and Bb chords make sense, but the E in the final chord doesn't; so in this case, we just recognize it as a Picardy third, which just means we make the final chord major even though we expect it to be minor.

As for why it works so well: in the late nineteenth century, composers started playing around with just moving major triads around (we call this "planing"), and this is one of the sounds that stuck. In short, it's just lots and lots of major.

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    Those three chords have been used for endings in pieces in C major. It's rather like the Ab heralds a key change from C to Db. Often wondered what this cadence should be called.
    – Tim
    Commented Apr 19, 2017 at 8:04
  • The chords are actually borrowed from the tonic minor scale (Cm). They are native to that key.
    – 02fentym
    Commented Apr 23, 2017 at 5:47
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    There's really no need to justify them by 'borrowing'. Every chord is diatonic in some other key. We're in THIS key, C major. 'Borrowing' only becomes relevant when a chromatic chord is used as a gateway to another key. Otherwise they are 'chromatic chords in C major' and require no further excuse.
    – Laurence
    Commented Dec 20, 2018 at 14:59
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In a more modal sense, this progression comes to rest at C, not Ab, and it is commonly used this way in rock music. You will find it reversed in All Along the Watchtower and used in both directions under the electric guitar solo in Stairway to Heaven.

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I'm still thinking about/searching why they work, but Paul McCartney uses them in "Uncle Albert/Admiral Halsey" right after, "Hands across the water, hands across the sky" before he repeats that line, there is this progression: F - Ab - Bb - C. It sounds really good!

I still don't know why the Ab works in the Key of C, but I know how the Bb works, but not the details of why (theory). In any Major key, normally the vii chord is diminished. In the Key of C, the B would be diminished (Bdim). However, in harmonic theory and practice, you may, in a Major key, take the vii chord, flatten it and make it a Major chord. So, in the Key of C, you may take the Bdim, flatten it, and make it Bb (Major). The Beatles do this at the end of Hey Jude, where they repeat the "na na na na's." The song is in the Key of F, so the vii chord is E-diminshed. In the Na na na repeats, their chord progression is: F - Eb - Bb - F (which is: I, bVII, IV, I).

David

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The accepted answer by Richard is very good, but here is another way of viewing it, without relying on borrowed tones:

Here are all of the tones present in the chords A♭, B♭, and C:

A♭ - B♭ - C - D - E♭ - E♮ - F - G.

They fit neatly into the F minor scale when you consider the raised 6th and 7th degrees of the melodic minor. The thirds of the chords in the progression are after all ascending through C - D - E. (If you include the D♭ when descending you have the full F melodic minor scale.)

We could consider this progression a half-cadence (ending on V) in Fm, but I agree with Richard's assumption that the tonic is C and it feels "major" because of the E♮

The implied scale is just the 5th mode of the F minor scale. (When limited to the melodic minor, this is also known as Mixolydian ♭6.)

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