Wrting a chord as III in conventional notation gives you, as far as I can tell, two pieces of information:

  1. The uppercase numerals mean it's a major chord, i.e., built by stacking a major third and a minor third
  2. The number 3 means that it is based on the third degree of the scale

However, Wikipedia writes the 3-chord in a minor scale as bIII:

Table showing roman numeral notation of diatonic chords in the minor scale

Why is this? Aren't the III and bIII the same in a minor scale?

Also, wouldn't the latter technically refer to the non-diatonic major chord rooted on the flattened third note in the scale (e.g., D major in the scale of C minor)?

  • III is often named V/vi as it's the major dominant of the chord based on the sub-mediant note in the major scale. The secondary dominant of Am in key C is E.
    – Tim
    Apr 28, 2021 at 7:52

6 Answers 6


If one is in a major key, then III is a major chord build on scale step 3. An example is in C major, the III is an E major chord, E-G♯-B (in some order.) The symbol ♭III in C major is an E♭ major chord, E♭-G-B♭ (in some order.)

If in a minor key, then the III chord is built on a flattened third so in C minor, III is E♭-G-B♭ (in some order). The term ♭III would be redundant.

  • Can I conclude that in ionian, lydian, and mixolydian modes the bIII is different from III, but in dorian, phrygian, aelian and locrian modes there is no difference? Apr 27, 2021 at 22:59
  • In keys where the three is already flat, you don't write it as bIII at all. Apr 27, 2021 at 23:24
  • That's what I thought, but the Wikipedia article linked in the original question says otherwise, for instance writing bIII for the third chord of a minor scale. Apr 28, 2021 at 11:14
  • This answer describes one of the two possible systems of chord naming. In my opinion the less useful one. See my answer.
    – Laurence
    Apr 29, 2021 at 10:24

I believe it varies between different people's/textbooks' conventions, but as I understand it, the Roman numeral itself indicates the degree of the scale, and the capitalization indicates major or minor. Any preceding accidental changes the root note from that degree of the scale (but the quality still stays the same, so this changes all the notes). This is called mode mixture because usually you're borrowing chords from a different scale, e.g. the parallel major or minor.

So if you're in C minor, III is E-flat, G, B-flat. If you're in C major, III is E-natural, G#, B-natural. But flat-III in C major is E-flat, G, B-flat because you're flattening the root of the III chord.

I think Todd Wilcox is using a convention where all chords are labeled based on the major scale from the key center, regardless of whether the original piece is in minor or major.


The bIII is your tip-off that this is a "borrowed chord".

"When we’re in a major key, we can “borrow” chords such as iio, bIII, iv, bVI and viio7 from the parallel minor key, which means the minor key of the same name. Of these chords, iv is the most common. Borrowed chords in minor keys are less common, but we can sometimes borrow the I and IV chords from the parallel major." -- from https://popgrammar.com/borrowed-chords/#:~:text=When%20we're%20in%20a,chords%20from%20the%20parallel%20major.

See https://tobyrush.com/theorypages/pdf/en-us/borrowed-chords.pdf for some interesting educational posters.

  • Wouldn't the V chord in a minor key be considered a borrowed chord? That one is really common! Apr 28, 2021 at 11:26

The Wikipedia user who posted that image has done so based on no references or practical experience, as revealed by reading the chat for that page.

The convention is that the harmonic minor scale is assumed, so in the key of C minor, i=Cm, iio=Do (D diminished), III=Eb, iv=Fm, V=G, VI=Ab, viio=Bo (B diminished). There's no need to write III as bIII and no need to write VI as bVI. If you want a Bb chord, the seventh degree of the natural minor scale, you would indicate that as bVII.

  • If the harmonic minor scale is assumed, then III = Eb+.
    – Aaron
    Apr 29, 2021 at 8:31
  • Besides Wikipedia, I've also seen this notation in other places, for example in Eric Haugen's music theory series on Youtube. It guess the difference comes from the different ways music is thought. Thanks for clearing up my confusion! Apr 29, 2021 at 11:36

You are right on the first two points...

  • Roman numeral indicates the scale degree
  • Upper case means major triad, lower case means minor (also include suffix o means diminished triad and + means augmented)

But the important thing is the scale in question is given by the key signature. And there is a sort of matching and alteration that can happen with the symbols depending on the key signature and the intended chord qualities.

Keep in mind there is a system that simply labels scale degrees with all upper case Roman numerals regardless of chord quality.

Also, you should give key signatures with Roman numerals for clarity.

  • a sharp, flat, natural prefixing the Roman numeral means a chromatic alteration from the key signature.

So... in C major, just III could mean simple the third scale degree, E or any triad rooted on E.

In case-sensitive style iii is used because the diatonic triad on the third scale degree is minor. If it were given as C: III the chord would be E major. (A small aside: that chord could be considered a secondary dominant to chord vi A minor. To clearly convey that secondary dominant sense, you would write C:V/vi. The secondary dominant is sort of the standard harmony analysis way, changing the letter case to alter diatonic from the key signature is more of a jazz way to label chords.) Finally, you could prefix the number with a flat - ♭III - and that would mean the E is altered from the C major key signature to become E♭. Conventionally this is conceived as "borrowing" from the key signature of C minor which has three flats A♭, E♭, B♭, and the diatonic triad in C minor on the third scale degree is E♭ major. So...

  • C:♭III is the chord "borrowed" from C minor E♭ major

If the music was initially considered to be in C minor, the key signature already includes an E♭ and so no alteration or prefix is involved:

  • Cm:III is the diatonic chord in C minor E♭ major

Notice the difference in symbols depending on whether you give the key signature before the Roman numeral symbols.

  • Thanks for clarifying the difference between case-insensitive and case-sensitive notation. Can I conclude there is no difference between III and bIII in any minor scale? Apr 29, 2021 at 1:23

There are two systems of naming chords by Roman numerals. You can decide what key you're in and name strictly diatonically. In a major key, the tonic is I. And in a minor key it's still I. In C major, E major triad is III, in C minor E♭ major triad is III.

What, you might ask, about chords that include the 6th and 7th degrees of the minor scale? Do we name according to the natural, melodic or harmonic form of the minor scale? All three often mix freely in the same piece.

Good question. And it's why I recommend the other system, one that aligns with the way we name intervals. The tonic chord of a major key is I, it contains the major 3rd above the tonic. The tonic chord of a minor key is i, it contains the minor 3rd above the tonic. All we need to know is what the tonic is. If it's C, C major is I, C minor is i. E major is III, E♭ major is ♭III. G is V, Gm is v.

If there's a good reason for the first system, it's yet to reach me. However, it gets used. So try to pin down which is in operation in any given discussion.

(What DO you call a G major chord in the key of C minor using the first system? It happens a lot...)


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