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"Which" diminished is it ?

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Estimable Member
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Here's a time-waster of a question, because it doesn't REALLY matter...I'm just curious.
When writing out chords for a song, how does one determine the "correct" name for the diminished (or dim7)?
There are only three diminished chords really, so they have duplicate names.

Writing out chords for a song in C major...if I need to use a Bdim, I would call it a Bdim rather than an Fdim or Ddim, mainly because Bdim is a triad naturally occurring in a C major chord scale (it is the 7th) this correct ?
HOWEVER, if in that C major song, I need to use an Edim7, why should I call it that, instead of a C#dim7, or a Gdim7, or a Bbdim7 ?...and if so, why ?

Same thing applies to augmented chords.....picking the "right" name has to be a theory-based rule...doesn't it ?

"A child of five could understand this...send someone to fetch a child of five !"--Groucho Marx

Joined: 21 years ago
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As you say, it's a quirky bit of theory, but here goes.

If you're using the notes B, D and F in the key of C it can "only" be a Bdim chord because the "F-chord" in the key of C is F Major, and the chord built on D in the key of C is Dm. Likewise, you couldn't really call an Edim7 in the key of C a C#dim7 because C# doesn't exist in the key of C. Nor does Bb. My logic falls over with the Gdim7 version. Tom can no doubt explain it all a lot better

Hope this helps.

A :-)

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Yep, there's a rule based on roots. You want the name to make sense in the context.

Let's say you're playing a Cº7, C-Eb-Gb-Bbb(A). All four can be valid names for the chord in isolation.

So you look at where the chord is placed in the progression. Let's say you're in the key of C - you probably won't give it an out-of-key name (Eb or Gb).

Next you look at the root motion - the chords immediately before and after it. The most common root motion is by fifths or fourths; if the chords on either side are F/G roots, it's probably a Cº7; if they're D/E roots, it's probably Aº7.

One big exception to that - I'm assuming that the º7 chord has a big role - that it's held for several beats or otherwise emphasized. If it's just a quick passing thing, preference would be given to a chromatic motion - G-Gbº7-F makes more sense squeezed into one measure; G-Cº7-F makes more sense if it's a whole phrase. (Edit for clarity: I don't necessarily mean G and F major chords - they can be any chord type, since it's just the root motion that matters)

Now if that didn't give you a clear name, there are only two more motions available - seconds/sevenths and thids/sixths. Chords tend to move in step a lot more often than they move in thirds - since chords are built on thirds, this motion leads to very bland music - you'll have two voices overlapping in just about every change (that's why the transition to a relative minor sounds so smooth - Am and C share two notes... but you wouldn't go Dm-F-Am-C-Em-G, or have any other long string of chords by thirds, or you'll put the audience to sleep!)

So if the chords on either side have roots B/D, it's probably Cº7, and if they're G/B, it's probably Aº7. Notice you need the chords on both sides to figure it out - moving in from a B root could lead to either name; it's by knowing the whole set of motions that you properly name chords.

The same thing is true of augmented chords - and other chords too. C6 and Am7 have the same notes; you need to figure out how the roots are working to decide on the right name.

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Topic starter  

Thanks for the responses.
My progression is from a 1928 oldie named "Makin' Whoopie", end of verse, to start of Bridge:

end of verse:
G7 C C/B
Edim7 Dm Fm6 C C/B
Edim7 Dm Fm6 C verse


"A child of five could understand this...send someone to fetch a child of five !"--Groucho Marx