Using Roman Numeral Notation with Notes in the Bass (not figured bass)
I have a situation where I want to use Roman numerals to express a chord progression. In the jazz Real Book, this would look something like this:
(key of A major)
A A/G A/F# A/E D
I imagine this looks familiar to many people here on this site. The above is easy to read, but what if I want to use Roman numerals instead? The below, using figured bass, would become complicated:
I I(4 2) I(?) I(6 4) IV
... or worse:
I I(4 2) vi(7) I(6 4) IV
Is there a proper way to write out a Roman numeral chord representation with a clearly chosen scale degree as the bass note?
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The only complication I see is whether you have indicated the G you meant.
You wrote a G natural...
(key of A major) A A/G A/F# A/E
...in the key of A major normally G# is used in the scale. If that were the bass tone A/G# the Roman numeral would be I4/2. But if the bass tone is really G natural that chord is properly V/IV.
The slashes get a bit confusing so I will use the actual word 'of' and bars to try to make things clear...
A: I | V4/2 of IV | vi7 | I6/4
Those would be the Roman numeral for the jazz symbols you gave.
Notice that I started with A: that's indicating the key. You really need to do this with Roman numeral analysis. Keep in mind a song may change key or even have an ambiguous key, in which case you should make sensible key change notations, or Roman numeral analysis may not be appropriate for music that isn't really in a key. Also, be aware of the various Roman numeral symbols for borrowed and secondary chords which bring in chromatic harmony. Things like bVII or viio/ii etc.
It's not a big deal, but the V4/2 of IV doesn't move to IV and is sort of non-functional for that reason. This may be one of those ambiguous key situation like I mentioned above. It's really just an A major chord with a Mixolydian flavored descending bass line. Roman numeral analysis symbols can be a bit cumbersome to describe that.
FWIW, proper figured bass is different from Roman numerals. Similar, but not the same. Best example case I think is the I6/4 chord. In the key of C the figured bass would be a notated G and the figure 6/4. Take note that the I of the Roman numeral system is indicating the tonic, but the notated G of figured bass is indicating a dominant tone. To super condense a bunch of history, the era of figured bass did not recognize that chord as an inverted tonic chord. It was a dissonant fourth above the bass which required resolution. Mixing a bit of old and new systems, when that dissonant (non-chord tone) fourth resolved by stepping down to a major third above the bass then the proper chord was formed and that chord was a dominant. Not sure this helps or makes sense, but my point is that Roman numeral analysis and figured bass are not the same.
This passage leading from A (tonic):
-> to the subdominant D
would be interpreted as (in relation to D):
I:
-> (V2 iii7 V46) IV
I think the answer is no, there is no "proper" way, no conventional way to do this. I've never seen it.
But I would write: I I/b7 I/6 I/5 IV, I have never seen anyone else do it this way, but for me it works.
It would be more correct to write VIm7 (or vi7 classical music style) in stead of I/6, but I/6 makes the movement in the bass more clear.
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