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These are the chords for some A minor scales:

Minor scales

For melodic minor, why does the F♯o diminished chord have a '♯' in front of the Roman numeral, and the G♯o diminished chord does not?

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  • In much music, the minor mode is treated as the union of these scales. The minor chord on step 5 is common though mostly as a key area. Often the v-i pattern will be used at non-final cadential points, reserving the V-i for the final cadence. The III+ is rare as is shown. The major IV sometimes occurs as it's used both for "color" and for giving a "modal" feel. Both ii and II (and II7) occur, especially at final cadences. My point is that all these need practice.
    – ttw
    Mar 18 at 3:03

2 Answers 2

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When analyzing a piece in a minor key, the VI chord is presumed to be on the 6 native to that key (signature), while the vii is presumed to be raised. Since the 6 in melodic minor is raised, its triad requires a # to differentiate it and make clear it's the raised sixth. While vii is presumed raised, one sometimes sees bVII when appropriate, for clarity.

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A sharp or flat precedes a Roman numeral if the chord root differs from the key signature.

That system is consistent except for chord roots of the seventh scale degree. Unfortunately, that is a mix of various assumptions.

If upper and lower case letters are being used, upper case VII is assumed to be a chord root on the subtonic, the scale degree one whole step below the tonic, and the chord quality will be a major triad. If the lower case vii is used, the chord root is assumed to be a leading tone one half step below the tonic, and the chord quality can either be assumed a diminished triad, or the o diminished symbol should ideally be given as viio.

While not part of your question, I dispute the "rare" label on the minor v and the augmented III+. Minor v is actually common. III+ is also fairly common, but you might be able to analyze that chord as some kind of non-chord tone action involving a dominant chord. Ex. root E with a G♯ and above a C moving down to B. E G♯ C being an augmented chord, which has some ambiguity regarding the root, or it's a dominant chord E G♯ with C and appoggiatura resolving down to B.

What the charts in your example do not show, but are probably more common uses of sharp and flat prefixes to Roman numerals are the neapolitan chord, which could be labelled as ♭II and borrowed chords. For example, in C major the submediant triad is labelled C:vi, but borrowed from the minor the submediant will be labelled C:♭VI.

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