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Is using a secondary dominant in a chord progression similar to using modal interchange? I hear the use of bVI chord in a major key quite similar to a V/V chord (It might just be me). How are they different?

  • In key C, V/V has notes D F# A C. bVI has Ab C Eb Gb(F#). That's two notes the same, like tts. Did you mean bVII, with Bb D F Ab? – Tim Jan 10 at 7:59
  • No, I meant bVI. They sound quite similar, and I only just learned about tritone substitution through one of this post's answers. Thanks for your response! – Jishnu Jan 10 at 12:44
  • @Tim V7/V is D F# A C, but V/V is only D F# A... no shared tones with bVI. Not sure if the OP meant that specifically – Michael Curtis Jan 10 at 16:38
  • Tritone substitution would be a lot clearer if the 7 was put on the chords. – Michael Curtis Jan 10 at 16:43
  • Yup. It's stronger to have the full tritone. But half of it still works! – Laurence Payne Jan 10 at 17:54
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A secondary dominant, by definition, leads to the dominant. Thus V/V, in key C, it's D(7) leading to G. Going anywhere else, that D or D7 isn't going to be a secondary dominant.

The bVI you quote is, in key C, Ab7, which can be considered as the V of a new key, C♯/D♭. A common key change in many songs. If it came with G straight after it, it could be considered as a sort of strange cadence, either imperfect if it stopped after playing G, or perfect, (G>C) with the A♭ as a red herring.

They both contain a 3rd and b7 of each other, but swapped over, producing a kind of tritone substitution effect, which may be what you're catching. That said, their functions are different.

  • That's exactly what was puzzling me. Thanks again! – Jishnu Jan 10 at 12:46
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You've discovered the 'b5 substitution' or 'tritone substitution'. Yes, in the key of C, Ab7 can function as V of V. It shares a tritone interval with D7 - the notes C and F# (Gb).

This illustrates the general principle that any dominant function chord can be substituted with a chord rooted a b5 higher. If both notes of the tritone are present (i.e. if it's Ab7 rather than just Ab) the dominant effect is heightened, but just as G (without a 7th) is still the dominant of C, Ab can still sound like the dominant of G.

On a more pragmatic level, a 'dominant' can be rooted a 5th below (G7 to C) or slide down from a semitone above (Db to C).

Common Practice harmony used this idea in the Augmented 6th chord, normally used as a pre-dominant. Jazz-influenced harmony uses the b5 substitution freely.

I'm not a great fan of 'Modal Interchange'. It does little beyond hanging a 'permitted' label on some 'safe' chromatic chords. We have a good, firm functional reason for tritone substitutions!

  • This has cleared things up for me, will experiment with tritone substitution to find out more. Thanks for the response! – Jishnu Jan 10 at 12:50
  • I don't mean to be argumentative, but why do you dislike the term 'borrowed chord?' I just figure it means a kind of harmonic "coloring" or in the case of borrowed v or bVII or sort of "modal" cadence. Do you think some people believe "borrowed" means a special (Reimann-like) function? – Michael Curtis Jan 10 at 18:30
  • I just don't find 'borrowing' a particularly useful concept. It often seems to come from an obsession with diatonic-ism. "This naughty chord isn't diatonic in the key we're in! Oh, it IS diatonic in some other key. That's alright then!" That's not an analysis. It's an excuse for not investigating what the chord is doing. If anything - chords don't have to be functional. Maybe it's just the chord next door. – Laurence Payne Jan 10 at 18:39

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