How to choose between Cdim and C#dim?

Question

I have noticed that there is a common chord progression that exists in a few variants: C, C#dim, Dm, G7. It's often changed by changing Dm to D to give the progression a little bit more spark. However, it's also fairly common to change the C#dim to Cdim. I am a bit surprised about how similar they sound in context. Changing a whole chord half a step usually have a pretty dramatic impact on a chord progression, but here it's more like just adding a 7 or something.

Is there any theory behind this? And how should I reason about which I should pick, apart from the obvious "what sounds best".

I wanted to describe the above progression with function notation, but I thought I'd just mess it up. Please tell me how to write it properly. I know it should be something like I, X, ii, V7 but what should X be?

As requested in comments, here are four samples to show how little impact it has:

• C, Cdim, Dm, G7
• C, Cdim, D, G7
• C, C#dim, Dm, G7
• C, C#dim, D, G7

Answer 1

The similarity is likely due to the fact that both can be heard as passing diminished chords formed by chromatic voice leading. The C#dim is formed from an upward motion (C - C# - D), while the Cdim is formed from two downward motions (E - Eb - D; G - Gb - F).

As the other answerers have noted, the C#dim also has a functional interpretation (as an incomplete dominant). The use of one and the same sonority as a functional chord and a by-product of voice leading is a unifying technique used in many great works (e.g. the Tristan chord).

Answer 2

I wondered about this myself, actually. The following is my opinion as to the workings of these chords in tonal harmony.

The C♯°7 leads very well to Dm (vii°/ii). It is fundamentally a different chord than the I, and it resolves up to the Dm. This chord is much more "directional"; it can't really return to the I. It also can be seen as a substitute for A7 (or A7♭9).

The C°dim7 (actually, I'm going to call it E♭°7 to show the leading tone motion) is more of an altered I chord. Its notes actually tend to resolve down to the notes of the Dm. It can even go back to the I chord, and it's not as pointedly fixed towards Dm. This isn't really a substitute for A7, only in function in its resolution to the Dm chord. When we look at Em-E♭°-Dm-G, we can see that E♭° seems to be more of a tritone substitution-y sound, or even A♯9 (♯11 works too, respelled). Certainly more "outside" than C♯°.

That was a bit of a rant, but I think the most important things can be summed up with these:

• The two serve the same function
• C° (or E♭°, in context) sounds more "jazzy"/"outside", where C♯° is more clearly functional/classical.
• C♯° is more directional

Answer 3

Is there any theory behind this?

C, Cdim, Dm, G7

This is the true passing diminished chord. True, meaning there isn't a functional label to apply to it using Roman numerals. In fact in Roman numeral analysis you could leave the chord unlabeled to make clear it's only passing motion: I ... ii V7.

C, Cdim, D, G7

This could possibly be called passing motion. But as @user59242 pointed out in their now deleted answer, the chord could be treated like a full diminished seventh chord and respelled...

Cdim: C Eb Gb (Bbb) respelled to F#dim: C Eb F# (A).

That's fudging things a bit because it changes a diminished triad to a diminished seventh chord which isn't the exact OP. I still think it is valuable to point out this possibility, because you can find this kind of harmony.

With the respelling and filling out of seventh chords we get C, F#dim7/C, D(7), G7 where both the F#dim7 and D7 have the same function leading to the G7 chord. It's clearer with Roman numerals: I viio43/V V7/V V7. It's a chain of dominants!

C, C#dim, Dm, G7

In RNA, I viio/ii ii V7.

...and...

C, C#dim, D, G7

In RNA, I viio/ii V/V V7.

In both cases the diminished chord is simply the leading tone triad of the D chords.

...I am a bit surprised about how similar they sound in context.

Indeed, it's surprising to hear all the options. Someone might think that change would not work. But there really is a simple explanation. From a functional harmony standpoint the variable part of the phrase is identified as pre-dominant harmony. Lots and lots of chords can fulfill that pre-dominant role. You can abstract that harmony to this: I ... V7. In essence we are saying the phrase is just a move from tonic to dominant and the two pre-dominant chords are mere details. All four phrases are roughly equivalent.

I would point out this one important distinction. Something like C, F#dim7, D7, G or I viio43/V V7/V V. In that case the chords are similar, but the tonal center shifts to G. Just keep in mind the movement to the G chord can treat G as the dominant of C major or as a new tonic in G major depending on the exact chord details. Functionally these are not equivalent.

...How to choose between Cdim and C#dim?

They all can sound good. That's really an artistic choice.

Answer 4

We're looking at I, X, ii, V, I, where X is a choice of pre-pre-dominant chords. Something that will work as a dominant of ii.

Basic 'circle of 5ths' bassline would be C, A, D, G, C.

We know that C#dim can function as a rootless A7. So a good bassline would be C, C#, D, G, C. Or, on a more pragmatic basis, if you want to get from C to D, what could be more obvious than putting C# inbetween?

We know that any dominant-function chord can be substituted by one rooted a tritone above. So another good bassline would be C, E♭ D, G, C. (We could extend this idea to C, E♭dim, D, D♭dim, C.)

Listen to just the first two chords, you'll hear quite different things. Listen to the whole sequence, yes, they all end up in the same place.

Which one to use might be dictated by the melody. It may be purely an artistic choice. There isn't always a 'right answer' in writing music!

Answer 5

How to choose: which V-I motion do you wanto to imply? Depending on what your "V" chord is, you choose the corresponding substitute dim chord.

I use a simple way for looking at dim (or dim7) chords: one of the notes works as the third of some major - or major 7th i.e. dominant 7th chord. This logic comes as a consequence from the fact that you can substitute a dominant 7th chord with a dim7 that includes the third of the chord. For example, you substitute a G7 with Bdim7 (or Ddim7 or Fdim7 or G#dim7). There's no single correct answer - any interpretation works.

So, for your example, the C and then Cdim or C#dim question. Following the rule of thumb I presented above, as which dominant sevenths can a Cdim work? :

• (1) Ab7 (imaginary tonic being Db or Dbm)
• (2) B7 (imaginary tonic E or Em)
• (3) D7 (imaginary tonic G or Gm)
• (4) F7 (imaginary tonic Bb or Bbm)

In the context of the key of C major, interpretations 2 and 3 feel logical to me. For example if the chords are like, C - Cdim - Dm, you could treat the Cdim as a B7 secondary dominant intending to push to E7, before the harmony changes its mind and takes Dm instead. As melody or "solo", try playing whatever you would play on top of a B7, on the Cdim.

Then in the same way, let's see as which dominant sevenths can a C#dim work? :

• (1) A7 (imaginary tonic being D or Dm)
• (2) C7 (imaginary tonic F or Fm)
• (3) Eb7 (imaginary tonic Ab or Abm)
• (4) F#7 (imaginary tonic B or Bm)

In the context of the key of C major, interpretation 1 is the most natural for me. I see the chord progression C - C#dim - Dm as almost the same as C - A7/C# - Dm.

I think this is a very simple and practical way to deal with dim chords, interpreting them as being different-tasting dominant chords. Or offering ways to jump to different tonal interpretations. It's kind of a game of illusions and tricks - where's the tonic now? Are you sure? Where is it now? :)

Answer 6

The biggest obvious difference between the C and C# option is the baseline will be changed. So will the way melody notes will sound over the chord.

Typically, chord progressions are not played in isolation, so common factors in choosing exactly which chords are used are how the bassline might go and where the melody is going.