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Let's say we have a chord progression I-ii-iii in key of C major so Cmaj-Dmin-Emin. If I were to share this chord progression with someone, without mentioning the key, and they decide to play this in the key of C minor, then they would run into trouble since that key doesn't have a first degree major, second degree minor, or third degree minor. Does that mean that chord progressions only work when in compatible keys? Is it up to people to guess the right key when looking at chord progressions in roman numerals?

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    We should note that not all Roman-numeral traditions use case to show the quality of the chord. In many places, chords are still labeled with all capital letters: I, II, III, etc. – Richard Jul 8 at 14:31
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The quality of the chords given by upper/lower case hints at the mode, major or minor.

I ii iii a major chord on the first scale degree, and two minor chords and the second and third degrees fits the pattern of major keys.

i iio III minor, diminished, major chord qualities on the first three scale degrees matches the pattern for minor.

But some combinations are ambiguous. IV V can happen in either major or minor. You should give the key along with symbols, like Cm: i iio III to make things clear. Giving the key along with prefixing accidentals to the Roman numerals will tell unambiguously what the chord root are.

Technically, just listing I ii III is confusing. In a major key that III means a major triad on the third scale degree, a major third above the tonic, in C it would be an E major chord. III being more typical of minor keys suggest it's supposed to be a borrowed chord. But in minor III is a minor third above the tonic, Eb. To indicate that properly borrowed chord in major you must use upper case for a major chord and also prefix a flat to show the lowered mediant of minor. C: I ii bIII for C dm Eb triads. In other words, C: bIII and Cm: III are the same chord, Eb major. To make the root clear you need the key and if borrowing in major, the accidental prefix.

...I-ii-iii in key of C major... If I were to share this chord progression with someone, without mentioning the key, and they decide to play this in the key of C minor

That seems like an odd scenario. Given I ii iii why would someone play C minor? But if you did - Cm: I ii iii - it means roots C, D, and Eb and respective chord qualities major, minor, minor, for C Dm Ebm. That doesn't sound like minor key harmony.

Is it up to people to guess the right key when looking at chord progressions in roman numerals?

No. It's up to the person writing the analysis - or shorthand chord notation - to do it correctly. Simply things like I IV V I are obvious, but add a key for anything that might be unclear.

  • The key label gives the palette of scale degrees
  • Roman numerals give the scale degrees used for chord roots
  • letter case gives major/minor quality
  • o/+ show diminished/augmented quality
  • accidental prefixes alter roots from the key label
  • Arabic numerals given last indicate intervals above the bass (not root), for purposes it tells chord extensions and inversion
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Yes. In Roman Numeral notation, the Roman Numeral used corresponds to the note number counted from the tonic. Individual notes are usually given with Arabic numerals (usually with a caret on top, but I don't know how to generate these easily.) So in the key of C Major, the notes (in order) C-D-E-F-G-A-B or 1-2-3-4-5-6-7; in Eb Major the numbers 1-2-3-4-5-6-7 correspond to Eb-F-G-Ab-Bb-C-D. Thus the numbers are independent of the key.

Roman numerals are similar; these are used for chords rather than notes. The Roman Numeral (mostly) corresponds to the root of the chord. In C Major, the (natural) chords are C-d-e-F-G-a-b0 where upper case numbers are used for major and augmented chords and lower case for minor and diminished chords (augmented chords get a superscript + and diminished chords get a superscript 0). The corresponding RN are: I-ii-iii-IV-V-vi-vii0. The advantage is that the number-based notation is identical in every Major key.

Minor keys have the complication that notes 6 and 7 have two forms, a lower (I'll call this natural with no mark) form and upper (which I'll call the raised form with a # sign.) Thus the natural C minor scale is C-D-Eb-F-G-Ab-Bb; which has chords i-ii0-III-iv-v-VI-VII. If a note be raised in chord in a minor key, the chord with that root gets its symbol changed: iv to IV with step 6 raised or v to V with step 7 raised (also VII to vii0 with 7 raised.)

It's a very flexible system. There are some additions which are covered in most harmony books. One useful is the shorthand of adding a "7" to the RN to denote a minor seventh (from the root) being added. So a G7 chord in C major is V7 (the 7 is supposed to be a superscript). Also, some figured bass notation is useful; V (in C major) denotes G-B-D with V7 denoting G-B-D-F. Similarly, one can represent chords with accidentals like C minor in a C major context by changing case from I to i. Chords with the third in the base likd E-G-C would be notated as I6 (superscript 6).

There are resources on the 'NET and harmony books that go into this more deeply.

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Answering what I think you're asking: the point of using RN is that it applies to any key. So in key C major, C=I, G=V. In key E major, E=I, B=V. And so on. That's the point.

Changing from a major key to a minor key, and expecting to use the same RN, it won't work.In key C major, I=C, and in C minor, i=Cm. After that, there are no real likenesses - with the exception of G=V - but even that could be Gm=v.

A piece with as you suggest I, ii, iii will not translate into the parallel minor key mainly because the harmonies are totally different. i, iio, III. As in C, Dm, Em isn't anything like Cm, Do, E♭. But, i, iio, III will be the same prgression in whatever minor key.

So, yes, I suppose people just have to guess what key RN might signify, as 'I' could be any of the twelve major chords - thus tonics of their personal key. In major, V/V will always be the chord built on the 2nd note of the scale, but major, as it's the 'dominant of the dominant' - a secondary dominant.

It may be of interest to check out Nashville Number System. Similar but not the same.

EDIT: in light of changes made to the question - giving someone 'I - ii - iii' generally speaking would mean 'root major - supertonic minor - mediant minor. Using both capitals and lower case would indicate which were major, which were minor. Any key would suffice for that second person, not knowing the original, because I - ii - iii works the same in any key. They would not consider that C - Dm - Em transfers to Cm - Do - Eb.

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Letter case does the job. Upper case always for major or augmented chords; lower case for minor or diminished. In analysis, usually you set the tonic center before the roman numerals flux, sou you'll always know that c: i is C minor and c: I is C Major (also, when modulating, you inform again the tonic center).

Changing the mode means you need to change the progression as well, some chords you be just fine by exchanging the letter case, but some will need a special attention to transform from or into a diminished.

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Yes, if you go from major to minor you likely need to change the chords. In this case: Cm Ddim (or Dm7b5) Ebmaj . It may still "work" or sound good, but it will be a different progression.

Be mindful of:

  1. dominant chords: you may or may not want to preserve major (or dominant seventh) chords in the minor key. If in your progression contained G(7) chord, in the minor key you could play either G(7) – more "classical" sound or Gm. Try both and compare the sound.
  2. the scale you use will change the chords you derive from it. Frequently used minor scales are:
    • aeolian: (c d eb f g ab bb), making the progression Cm, Dm(7)b5, Ebmaj
    • dorian: (c d eb f g a bb) Cm Dm Ebmaj
    • harmonic minor (c d eb f g ab b): Cm(maj7), Dm(7)b5, Eb(maj7)#5

There are many other scales to try, including melodic minor, phrygian...

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    I question the statement 2. Aeolian isn't necessarily the go-to minor. harmonic and melodic notes get used probably more - particularly the leading tone. – Tim Jul 8 at 13:17
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    This gives an error, or not enough explanation. first three triads in C minor are Cm Ddim E (flat) major. – Michael Curtis Jul 8 at 13:48
  • @MichaelCurtis - in key Cm, that D chord can be D minor - from the melodic notes. – Tim Jul 8 at 15:00
  • @Tim, sure. And it could be a secondary dominant to V, etc. I'm list the diatonic chords of the key signature. But the more important point is about the III chord. – Michael Curtis Jul 8 at 15:50
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    @MichaelCurtis E was obviously a typo, it should be Eb. Thank you for spotting! – user1079505 Jul 8 at 21:17
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Michael Curtis, Tim, and Rodrigo B. Furman are correct, and the accepted answer is misleading. You write:

If I were to share this chord progression with someone, without mentioning the key, and they decide to play this in the key of C minor, ...

It's not possible to play that chord progression "in C minor" because it is a major-key progression. That's why the I chord is in upper case. You can adapt it to a minor key, but the only way to do so is to change the chords, which means that you are no longer playing the same chord progression.

There are only twelve possible realizations of that chord progression (ignoring voicing differences): one for each of the 12 major keys.

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