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I'm transcribing something in C. The chord notes in a particular bar are, in rising order, C#, G, B, E. As I see it, there are three choices - Em6, A9 (no A), and C#m7b5. None of them seems to be a good name for the chord. It comes between a C bar and a D bar, so maybe C#m7b5 is best, as a sort of V/V/V. What criteria are there for naming it?

  • Subjective, and Western theory is still archaically wedded to conventions in a way which doesn't provide a unique description for every situation. I'd intuit the root (subjective!), strongly considering the bassline. I've seen plenty of "criteria" which struck me as people making rationalizations for falling upon prejudice. I believe that jdjazz makes a good point about trying to match a familiar progression -- though it relies on familiarity, it can make things easier conceptually for performers in several ways. – Epanoui Nov 12 '17 at 0:13
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My take is that function wins out as the most important criterion. The two best candidates are: C#m7b5 (or C half diminished, C#ø7) and A7. When we look at the function, I think A7 is the stronger choice.

A half diminished chord has the primary function of leading into a V7alt chord. For example, if the progression were: | unknown chord | F#7alt | Bmin | then C#ø7 would be a great candidate for the unknown chord because it would complete the minor ii-V-i. (Note: if the song were modulating from C to D, then this might actually be a C#ø7, but that's not the case as you've pointed out.)

But given that the progression is | C | unknown chord | D |, I think the best candidate for the harmony is A7, because the A7 chord serves as a V to the D chord, as you've pointed out. If C is the tonic, then this is the start of a I-VI-ii-V progression (or maybe I-VI-II-V, depending on the quality of the D chord).

There are many ways to voice an A7 chord without changing the underlying dominant seventh/mixolydian harmony. It can be voiced with the 9, it can be voiced without the root, it can be voiced with the 13 instead of the 5, it can be voiced with the 4 alongside the 3, etc. I view these changes as stylistic--voicing the chord in these different ways doesn't change the underlying dominant seventh harmony, and in all cases, the A7 chord is still playing the exact same functional role.

So the question, "what's the underlying harmony?" requires an eye on the function, in my opinion. In any given chorus, an underlying chord can have many different instantiations which would add extensions, remove notes, etc. So the approach I take is to write the underlying harmony--I write what a chord sheet/lead sheet would show. To that end, I'll listen to how the same measure is played in other choruses, other versions, etc. In the next chorus, maybe there's an A in the root. In the following chorus, maybe the 13th is included. But if I notice that the C# is always in the root, then I might notate this as A7/C#.

Functionally, the other strong candidate for this chord would be C# diminished (which would serve the same role as the VI7 chord). But as you yourself have already noticed, the presence of the B note in the chord rules out C# diminished as a possibility.

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    Thanks for the insights. Funnily, as I wrote it out on first hearing, C#o got put in that place. It was only in consultation with the writer - who plays but doesn't know names - that the particular voicing came out. – Tim Nov 11 '17 at 16:48
  • There's no reason a half-diminished chord can't also lead into a tonic chord, though; there's no "rule" that half diminished chords only lead into V7alt chords. – Kyle Strand Nov 11 '17 at 19:39
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    @KyleStrand, I think you've misread. I didn't say there's a rule that half-diminished chords only lead to V7alt chords. This isn't a process-of-elimination argument. I have characterized the "primary function" and have used that to make a best selection between two candidate chords. – jdjazz Nov 11 '17 at 20:27
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    @KyleStrand, half-dim chords occur naturally on the 7th degree of a major scale and on the 2nd degree of a natural minor scale. They can serve three functions (diminished, dominant, and predominant). In a diminished function, they typically are a #ivø7 chord. In a cadence, they might serve a dominant function, typically as a viiø7 chord. Or they can serve a predominant function, as in a iiø-V7. You're proposing C#ø7 here, a #iø7 chord. This seems like a weird choice: it deviates from the normal uses of half-dim, & one could instead just characterize it as a standard I-VI7-II7-V7 progression. – jdjazz Nov 11 '17 at 20:40
  • I said "lead into a tonic chord", meaning that I'm considering D the I chord in this context, which would make it a viiø7, not a #iø7 (!). Without more context clues, I'm suggesting this is a modulation. Of course, if the next few chords indicate that G is the tonic, then this could also be a #ivø7 as you mention. – Kyle Strand Nov 11 '17 at 21:08
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If you subscribe to the theory that wants every chord to have a dominant relationship to the one that follows, we're looking to analyse the C# as a leading note to D. So this could be thought of as a rootless A9. THINK of it as that, but WRITE it for performance as C#m7b5, else the player will be tempted to add an A bass note. Same harmonic function but different sound, hence NOT freely interchangable!

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    Is there such a theory? Surely that can't be the case. Although an awful lot of chords are followed by their I, as in II V I it won't be every chord - we'd end up doing the cycle of fourths every time! – Tim Nov 11 '17 at 17:05
  • I said 'wants'. Functional Harmony doesn't always get what it wants! It can get very upset by, say, a bVII chord – Laurence Payne Nov 12 '17 at 17:29

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