Flats (b's) in chord progressions

Question

I found out that we had two (basic) chords intervals:

• Major scale chords: I ii iii IV V vi vii°

• Minor scale chords: i ii° III iv v VI VII

Here one can see some charts of chords from common progressions: Chord Progressions

So far so good... but then I found other people using progressions with flats in them... why?

I thought these roman numbers could represent any chord (any fundamental)

Examples:

• i – bVI – III – bVII

• i – bVI – iv – bVII

So why b's??? Where does that comes from?

Answer 1

There are three reasons to use accidentals in front of Roman numerals.

1. As noted in the question, standard classical theory tends to assume the accidentals of the scale are implied in the Roman numerals, whether in major or minor. (The one addition to this is that vii^o in minor is assumed to refer to the raised leading tone, while VII tends to refer to the lowered seventh scale degree.) So, in a C minor piece, a III chord is assumed to represent E♭ major, a VI chord is assumed to be A♭ major, etc. However, some jazz and pop music theory books start instead with a convention that the major scale is pretty much assumed as the standard scale. (This becomes useful, for example, when you have other potential modes other than just major and minor.) In that case, any piece based on C would assume the Roman numerals by default refer to C-D-E-F-G-A-B-C. Thus, if you wanted to label an E♭ major chord, you'd write ♭III. An A♭ major chord would be ♭VI, etc. (And occasionally when the mode is unclear, some sources will use accidentals with Roman numerals just to emphasize what's going on.)
2. Another use is to indicate modal borrowing. That is, if you're in C major, but make use of an A♭ major chord, that chord can be thought of as "borrowed" from the parallel minor key of C minor. Some books will just use VI anyway and assume you'll note from context that it's a borrowed chord. But others will put a flat in front of the Roman numeral to clarify that the root of the chord is lowered from the normal note of the C major scale, hence ♭VI.
3. Finally, there are some modifications to scale degrees that don't occur in either the standard major or minor scale. In a C context (either major or minor), there is no D♭ in the scale. But it's somewhat common to have a D♭ major chord appear sometimes (otherwise known as a Neapolitan chord). In that case, the accidental can be used with a Roman numeral to indicate a note outside the scale, as in ♭II. Accidentals can also sometimes appear with more exotic chords, like ♯ii^o7 indicating a diminished seventh built on the raised second scale degree.

Your second example ("i – bVI – iv – bVII") clearly seems to be an example of the first category. It is clearly a minor mode context, but the flats are added to clarify that the VI and VII are based on lowered versions compared to the major scale. The first example ("bVI – III – bVII") is weirder and I'd need to see context. I'd probably assume it's also an example of the first category I listed, but the author forgot the flat sign before III; otherwise, this looks like it might be a very odd progression.

Answer 2

Take a major scale. A scale is a run of notes and a major scale has a certain pattern to the notes intervals (intervals being the space between two notes). So in the key of C a major scale would be:

C D E F G A B (C again so it sounds like we resolved the scale, don’t leave us hanging on that B)

Now take that scale and build a chord above each note with a third (interval) and another third. But only use notes from the scale. This means sometimes you will be using a major 3rd and sometimes a minor 3rd.

A major third, for example, is from G to B. Or C to E. They are a set distance away. C to Eb is a minor third. G to Bb is a minor third.

The point is you want to stay in the scale.

So the first note is C. A third above that will be...let’s see...skip the d, because that would be a second of some kind ...so E!

Ok, so now we have a C and and E and if played together they almost make a chord. By definition we need three notes played together to make a chord. So let’s go up a third from E and we skip F and get a G. Alright now we have C E G and we get a major chord.

Do it again for D and we get D F A. Note that we didn’t use a F# (sharp) that would be a major third from D. Because F# is not in the scale of C. D F A is a minor chord. Listen to it.

If you continue to do this for the whole scale you will get a series of chords that are know as the diatonic chords. Which is a fancy way to say chords that are naturally in the key you are with no notes from outside the key.

Now to your question. Why would someone use a Bb chord of some sort in the key of C? Well...mainly because someone thought it sounded good. And there are more chords in the world than the ones from the key you are in. These are called borrowed chords meaning they are borrowed from a related scale. Perhaps the related minor scale, or another mode. In this case it is borrowed from C minor which will have a Bb major chord.