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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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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

  • Af C Ef
  • Bf 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 Af and Bf 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 Af and Bf 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 Apr 19 '17 at 8:04
  • The chords are actually borrowed from the tonic minor scale (Cm). They are native to that key. – 02fentym Apr 23 '17 at 5:47
  • 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 Payne Dec 20 '18 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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