How does Gdim7 fit in C# minor?
I'm pretty new to music theory. This song is actually the reason why I even started reading about theory, but I still can't comprehend this chord progression.
I think this is in C# minor, and this is the chord progression: C#m7 Gdim7 F#m7 B7sus4 C#m7
What I don't understand is the Gdim7 chord. Key of C#m doesn't have a G chord, but it fits in the chord progression somehow. What's the technical reason behind the flow of this chord?
Also, I'm looking at this from the composer's point of view, and I'm just very confused as to how he ended up writing this progression in the first place. It doesn't seem to follow the typical "pattern". For example, I've been using hooktheory to recreate this, but it doesn't provide an easy way to get the Gdim7 chord under C#m, where the G is the root note.
Thanks.
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G#dim7 is a rootless inversion for G7b9, making it a nice substitution for this type of chord. The b9 is an altered tension that goes well before a minor chord (look for the altered dominants in a minor 2-5-1).
The G#dim7 kind of acts as a G7, which itself is a tritone substitution for a C#7. C#7 is the V7/II - secondary dominant for the II chord, F#m7 - in the key of E major (C# minor). In other words, C#7, G7, G7b9 and G#dim7 are all good, flavored ways to precede a F#m7 chord.
It's also interesting that G7 is the secondary dominant chord resolving to B7, so it reminds something resolving late. The B7, in this case, is actually a Bsus4. The 4th replaces the 3rd of the chord, making it neutral from a major/minor point of view. Good way to keep that major color from messing up with the song's minor color. I believe this minor flavor is a part of the reason why that G# fits the song so well.
This is a good example of a non-dominant diminished chord with a diatonic function (i.e. resolving to a diatonic chord). Note that often diminished chords function as dominants. This is the case when the root of the diminished chord is the leading tone to the root of the diatonic resolution chord.
However, in your example this is not the case because then the diminished chord preceding the F#m7 chord should be a Fdim7 chord. The non-dominant diminished chord in your example works because - as also pointed out by Patrx2 - it has more than one tone in common with the resolution chord. A diminished chord moving down a half step is a very common device in jazz progressions. If you replace the C#m7 chord by its relative major chord Emaj7, the progression becomes a well-known jazz cliché:
|| Emaj7 | Gdim7 | F#m7 | B7(sus4) ||
which is analyzed as
|| Imaj7 | bIIIdim7 | IIm7 | V7 ||
with the bIIIdim7 being a non-dominant descending diminished chord.
I wanted to add some information to the already-good answers, about how to properly "spell" this chord.
Because chords are made up of stacked thirds, a seventh chord built on G will have to have some form of the notes G-B?-D?-F? (I'm using '?' to represent a yet-to-be-determined accidental). In the case of a Gdim7, these would become G-B♭-D♭-F♭. But we know that in our key (C♯ minor) we call the D♭ and F♭ C♯ and E. Yes, these pitches are "enharmonically equivalent," but the Gdim7 chord has to call them D♭ and F♭, since it has to have a stack of thirds.
Fortunately, though, dim7 chords are "symmetrical" -- we can start the chord on any one of its four pitches, and it has the same shape. That means we can pick any of its notes as the root. For example, if we pick E as the root (Edim7), then the chord has to contain E-G?-B?-D?, which, in order to make a dim7 chord, works out to E-G-B♭-D♭. That's better, but we've still got that pesky D♭ in there.
You can probably see where we're going next. By starting the chord a third lower, on C♯, the notes in the chord end up being spelled as C♯-E-G?-B?. In order to make this a C♯dim7, we need to make it C♯-E-G-B♭, which is a much better fit in our key. This is why I would recommend (as did Patrx2) calling this chord a C♯dim7, or a i dim7. I haven't used hooktheory before, but perhaps a i dim7 is easier to find than a ♭IIIdim7?
A little trick to remember all of this: regardless of the accidentals and the intervals involved, almost any chord (in western, common practice music) will be built from stacked thirds. That means they have to contain adjacent notes from a sort of "circle of thirds" scheme. Triads will contain three adjacent notes, while seventh chords will contain four adjacent notes.
A? - C? - E? - G? - B? - D? - F? - A?
Note how the circle wraps around on itself (it ends where it begins). Also note how I'm careful to not specify which version of a note I'm using. That will be determined by a number of factors including the key, and the desired quality of the chord.
Maybe because Gdim7 is the same thing as C?dim7? That means that, between the C?m7 and the F?m7, two notes are going to be held from the first chord, mainly C? and E, and there is going to be descending chromatic motion in parallel minor thirds between G? and B in the C?m7, and F? and A in the F?m7, because the remaining two notes of the C?dim 7 are G and B?.
That's to say that the diminished 7th chord is formed by chromatic passing motion between the first and 3rd chords of the sequence.
Edit: I'll amplify a bit. It's actually quite subtle because C? and E are common to all three chords, so the change of root a fifth downwards (from C? to F?) is being handled strictly by the chromatic parallel thirds moving the distance of a whole tone down.
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