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There's a song performed by Mimi Fox called "B-flat Blues" (as part of a True Fire course she presented).

The Bb major scale is:

Bb, C, D, Eb, F, G, A

This would suggest (based on my understanding of roman numeral analysis and how chords are constructed from the scale they're played within) that the chords that were applicable would be:

  • Bb maj 7
  • C min 7
  • D min 7
  • Eb maj 7
  • F dom 7
  • G min 7
  • A half-diminished 7

With this in mind, I'm trying to understand why she plays specific chords (e.g. why a Bbdom7 rather than a Bbmaj7, and why she plays Edim7 which isn't even in the Bb scale - Eb is but not E).

Is there a music theory reasoning behind the choice of chords played (which I've listed out below, exactly as they are played in the song, so of course you'll notice repeated chords listed):

  • Bbdom7
  • Ebdom7(add9)
  • Bbdom7
  • Fmin7
  • Bbdom7
  • Ebdom7(add9)
  • Edim7 (1st inversion)
  • Bbdom7
  • Gdom7
  • Cmin7
  • Fdom7
  • Bbdom7
  • Gdom7
  • Cmin7
  • Fdom7

The chord progression also looks to be a mixture of a 1,4,5 and a 2,5,1,6 and I'm not sure why it changes half-way through. Is there any music theory reasoning behind the change in progression?

  • Dominant 7th chords are often used instead of their diatonic counterparts in the blues form, and ii - V progressions are also frequently introduced (especially in jazz blues) to spice things up a bit. The B♭7 - G7 - Cmin7 - F7 is a common turnaround derived from what are called the "Rhythm Changes" found in Gershwin's I Got Rhythm. This is a I - vi - ii - V progression that is often altered to I - VI - ii - V, leading back to the tonic (B♭ in this case). – David Bowling Feb 18 '18 at 15:38
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This is a standard jazz blues progression in B flat. The linked site explains several variations, but the one you mention is a very common one. Blues is not written in a major scale. The characteristic of blues is that it uses notes from major as well as from minor, and that it's based on dominant seventh chords [Unless it's a minor blues, then you shouldn't use notes from the major scale, and you use minor seventh chords, at least for the I and the IV].

Let me try to explain a few things about that progression:

  • Fm7 Bb7 is a II-V progression leading to the IV chord (Eb7)
  • Edim7 leads back from the IV chord to the I chord by chromatic movement (Eb - E - F), where the F is the 5th of the Bb7 chord.
  • G7 is the V chord for II (Cm7)
  • Cm7 F7 is a II-V leading back to the I (Bb7)
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You're starting from a false premise, that a key defines a set of notes that MAY be used. Not so. The 'scale of the key' and the diatonic chords formed from it are a framework, and that's all. We modify, extend, substitute etc. them all the time, and we don't need to justify a chromatic chord by modulation or 'borrowing' - that's just perpetuating the fallacy that they somehow aren't 'allowed'.

C, C#dim7, Dm7, G7, C or C, A7, D7, G7, C are common sequences, and neither of them imply a modulation away from C major. D7 is II7 IN C MAJOR, not V7 in some other key. Got it? You'll find real-life harmony much easier to deal with now!

Your particular question also brings up the topic of the 'Blues'. This is a style based around I7, IV7, V7 (Bb7, Eb7, F7 in the key of Bb). There you have two 'chromatic' chords for a start - Bb7 and Eb7. But it's still a 'Blues in Bb', no-one would sensibly argue that it keeps modulating to Eb or Ab! A simple illustration of what I said above. Non-scale notes are just fine, and are commonplace.

I'm having to explain this simple point far too many times in this and similar forums. Whoever's teaching diatonic notes and chords as a restriction rather than a basic framework, please stop it!

  • It's like teaching an art student to color in a landscape using only a basic box of crayons isn't it. This is all you can use, nothing else. I support your campaign. – skinny peacock Feb 18 '18 at 18:31
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The blues specifically 'breaks the rules'. Except those rules are not rules. They're our way of trying to pigeon-hole and explain some of the things that happen in music.

You ask about why Bb7 and not the diatonic Bbmaj7. Well, the sound of the latter is too dreamy to be the main chord in blues. Making it into Bb7 sounds so much more ballsy.

The same goes for the IV chord. In blues, dom7 sounds more raw. Make it into dom9 is even better. Not usually written Eb7 add9, just 'Eb9'.

The dim in there could easily be called Bbo. In fact, if it's supposed to be Eo in 1st inversion, really it ought to be labelled Go. But as Bbo, it 'belongs' better to Bb. That will make some happy - those who think diatonic is gospel (small g)!

  • Funny thing, if I was going to tag Bbmaj7 and Bb7 as 'dreamy' and 'raw', I'd put them the other way around! – Laurence Payne Feb 20 '18 at 10:18
  • @LaurencePayne - you play a lot of dreamy blues? And 'Misty' sounds raw at the start? – Tim Feb 20 '18 at 10:35
  • With no context, I hear Bbmaj7 as pleasantly astringent, Bb7 as more mellow. But yes, either can be used in music of many styles! – Laurence Payne Feb 20 '18 at 10:39
  • @LaurencePayne - another factor, of course, is the manner in which each is played. Although an aggressive Bbmaj7 lacks balls. There again - 'It's not Unusual' says otherwise. – Tim Feb 20 '18 at 10:46
  • Yup. To consider another chord, in the score of 'West Side Story' Bernstein shows us how an augmented 4th interval can be cool, yearning, ominous... A reminder to us - and particularly the guitarists among us - to remember that chords are a collection of intervals and voice leadings, not self-contained objects. – Laurence Payne Feb 20 '18 at 11:18

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