What's the difference to a player between dim7 and 7b9?

Question

I'm trying to figure out how best to write a lead sheet for a player given a certain chord progression as an example:

shortchord.org/RqxRx (disregard the automatic names this app gives to the chords)

It seems like I have two options for how to write chord symbols for this progression, with the difference being in how to represent the second chord.

I can see naming this progression in terms of chord function, where the second chord is a secondary dominant of Dm7, as:

Cmaj7 | A7b9/C# | Dm7 | G7

( I | V of ii | ii | V )

Or, I can see naming the chord to be more accurate about which notes this specific voicing contains:

Cmaj7 | C#dim7 | Dm7 | G7

Both describe the rising chromatic bass note from C to C#, but one describes the dominant relationship, while the other specifies the dim7 chord structure without adding the implied A.

My question is, which is easier for a player to understand? Or, at least, what is the difference between what's communicated to a player in each approach?

Answer 1

The difference is in function. The A7b9 is a dominant chord leading to Dm. The C# dim would imply a passing chord between Cmaj and Dm.

From the standpoint of improvisation, if the chord is really really the V of Dm then I'd prefer your first rendering:

Cmaj7 | A7b9/C# | Dm7 | G7

This would imply a particular scale - perhaps a mixolydian scale with a flatted ninth or the fifth mode of (d) harmonic minor. As a player, I can still choose to play a (C#) diminished arpeggio idea or a diminished scale here if that's what I hear.

However, if I see this:

Cmaj7 | C#dim7 | Dm7 | G7

I'm more likely to think C# diminished.

If the diminished chord is used as a passing chord, as in:

|Cmaj7 C#dim7 | Dm7 G7 |

(two chords per measure) then I think the diminshed notation works better.

Answer 2

There's little to choose between them. The C#o doesn't contain an A note, which, if the chord was some sort of A, which it is, would imply moving in fourths. The C#o is much easier to read the first time at least; in the pit, at a fast tempo, I know which I prefer to see! I sometimes interchange these two chords - a bit naughty - but there's often times when only two or three notes from a chord get played anyway, so it may depend on what instrument, what other voicings of surrounding chords, etc. The bass would play C# regardless, so probably I'd prefer the A chord, if the objective was to move up in fourths, which probably is the case here. But for simplicity of reading, C#o.