Some songs refer to their key as specifically major or minor, but others use both the major and relative minor as key centers, in an even enough proportion where it can be hard to say whether the I chord or the vi chord is the tonal center.
Cmaj7 | Bm7b5 E7 | Am7 | Fmaj7 E7 | Am7 | Gm7 C7 | Fmaj7 | Em7 | Abmaj7 Bb7 | Cmaj7
This is in the key of C/Am, but it seems like it could be analyzed in Nashville / Roman numerals two different ways:
With C major as the I chord:
I | VIIø V7/VI | VI | IV V7/VI | VI | V V/IV | IV | III | bVI bVII | I
With A minor as the i chord:
III | IIø V7 | I | VI V7 | I | VII V7/VI | VI | V | #VII bII | III
I would most naturally write the analysis the first way, but I can see reasons for doing it the second way (jazz guys like to think in terms of II-V-I relationships, and that could make it necessary to call what I would normally call a VII-III-VI change a II-V-I in minor). What's the more effective way to communicate about this?
A related question: since jazz songs often have changes in tonal center, how should Nashville notation reflect those changes?