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I need help to understand chord intervals (not single note intervals) explains what the numbers I, IV, vi, III+ usually stand for in major chord theory.

But what's it like with a minor scale? What is the convention here? Let's say we describe a song written in A minor. How would I reference the chord progression a,F,E? Would vi,IV,III be valid? To me this feels more natural than saying "i,VI, V". If it has to be the latter, what about a mixture-song where you cannot really determine if it's major/minor? How can you be sure the other one knows what you're talking about? And what if there are multiple key changes? Does this notation then even make any sense at all?

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Roman Numeral analysis requires the key of what you are analyzing to make any sense. Based on the key you can handle any basic progression.

If you are in A minor, everything you are doing is centered around A so the progression would be i VI V for the progression Am, F, E. When outside of a major key sometimes ♭ and ♯ signs are used to denote when scale degrees are altered from the major scale so the progression may sometimes be i ♭VI V, but this is much more informal and gets screwy when looking at actual chords outside the key. You'll typically pick major or minor based on if the tonic chord is major or minor and use ♭ and ♯ signs to signify when the root of the chord has an altered scale degree so if the progression above had the Am changed to A major, the progression would be I ♭VI V in A major.

As you change keys you'll write in terms of the new keys so for example if you were modulating from C to G and you had the progression C, F, G7, C, Am, D7, G you would have this analysis:

     C   F    G7    C    Am    D7    G

C:   I   IV   V7    I    vi

                     G:  ii    V7    I
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Roman numerals directly relate to the key concerned. If they didn't, they would be less than pointless. Capitals (I, IV) relate to major chords, with a major 3 in them. Lower case (ii, vi) relate to minors, with a minor third.

So, in a minor key, say A minor, the Am is called i. If that key moves to its parallel, A major, then it makes sense to call the tonic 'I', due to it now being major. As noted by others, a chord altered will need a b or # to show this. Thus, in A minor, an F#m will be shown as #vi. The sharpened 6th degree chord formed in A minor (VI), but written vi as it's minor.

Somewhat similar to NNS, which is always worth a look at.

Depending on a bit of modulation, the same key can be followed, until there is obviously a sea change of a new key - with no prospect of a return imminent, at which point, the Roman numerals ought to reflect the new key.

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If a minor key IS established, it's useful to consider the tonic chord is i. Note that I said 'useful'. Like all 'theory', it's a set of descriptions. Use it when it helps. Use something else when that helps better. There was a recent discussion about the problem of the very common chord Dm6, or Bm7b5, acting as a dominant of E7. Absolutely tonal, and 'in-key', but beyond the scope of Roman numerals. Which doesn't make it 'wrong'. Theory describes, it does not command.

Roman Numeral analysis certainly 'makes sense', but it isn't useful in ALL musical situations.

There's a parallel with tonic sol-fa notation. A useful sight-singing method but a dead-end notation system.

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a, F, E would be described as i VI V if you're centered around A minor the major 5th would mean you're borrowing from melodic minor. Why would iv, IV, III seem valid? always think of the key you're centered around when speaking numerals.

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There is no convention. You need to know how chords are notated. This is independent of what scale you may be operating in.

Chords have root notes, these notes are certain scale degrees on the scale. So the Tonic chord has the Tonic for a root.

Then comes details on what type of chord you have. The four main types you are going to need to know are.

  • Major
  • minor
  • Diminished
  • Augmented

Now let us go trough what these mean.

  • Major - It has a Major Third and a Perfect Fifth
  • minor - This has a minor third and a Perfect Fifth
  • Diminished - Minor Third and Diminished Fifth
  • Augmented - Major Third and Augmented Fifth

A major chord build on a certain scale degree will have it notated with a Capital Roman Numeral.

A minor chord will be notated with lower case Roman Numerals.

An augmented chord will be notated with Capital Roman Numerals and a plus sign to indicate that this chord has an augmented fifth.

A diminished chord will be notated with lower case Roman Numerals and the degree sign.

Then lastly you have the inversion. The inversion basically concerns itself with what notes is in the bass.

If the root note is on bottom it is notated with 5/3 or none.

If the third is on the bottom then it 6/3 or sometimes just 6.

And lastly if the fifth is on the bottom then 6/4

You need to be able to look at any chord regardless of key and be able to tell what type of chord it is. Many modern pieces only adhere to the key loosely so the tonic chord can be many types of chord.

It is only in standard western harmony that the type of chords follows a certain pattern.

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