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On my older question Are the diminished chords not that common in modern popular music?, I got this answer:

"The half-diminished (or m7♭5) is usually suggested as the diatonic chord on the 7th degree in major keys and the 2nd degree in minor keys, and is fairly common in most styles of music."


That's right. Half-diminished sevenths are pretty common in popular music. However, if such chord is used, it would not be usually labeled as [X]m7♭5, but rather [Y]m6. (Believe me, I always have been labeling them as m7♭5.)

For example, in a circle of fifths progression in C major, they would usually write:

CㅡFㅡDm6ㅡEmㅡAmㅡDmㅡGㅡC

What's the reason here?

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    Don't see the relevance of your last comment. A lot of people use keyboards insead of pianos - all my 88 note ones have A at the bottom, but my 76 has E, and my 61s all have C. Also - to follow the circle, C-F-B-E etc. is considered up in 4ths (or down 5ths). Count the letter names. – Tim May 25 at 11:11
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    A bit of a nit-pick, but I think a better example would be E7 rather than E minor there – Some_Guy Jun 29 at 16:17
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Depends on the bass note. If the bass note of that third chord is in fact D, it functions as a mi6 chord. But if you put a B in the bass, it's a mi7(b5). Alternately, if you put a G in the bass it could also be a G9 chord. All about context. All those chords have somewhat different sounds, so choosing a different bass note for one chord can change the tone of the progression.

Edit:

Even though Dmi6 and Bmi7b5 have the same notes, they're different chords with different functions so I wouldn't recommend writing "mi6" to indicate "m7b5 a minor 3rd down" unless you truly don't care about the bass note. You could always write Dmi6/B if you want that half-diminished sound but aren't confident others will understand mi7b5 symbols.

Also, shouldn't Bmi7b5 usually lead to E7 or Ema rather than Emi if it's going to E in the bass, since usually its function is the ii of A?

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    If you put a different, extra note in the bass - or anywhere - it changes the chord and its name entirely! – Tim May 25 at 7:21
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If I understood the question correctly, you seem confused about the chords Dm6 (D F A B) and Bm7b5 (B D F A), because they have the same notes.

These two chords have different harmonic function, so you would distinguish the two of them like that. The vii usually acts like a dominant, so you'd find it followed by some kind of tonic, whereas the ii isn't a dominant chord, so you can follow it up with any other chord you want.

You can see that in the progression you posted:

C - F - Dm6 - Em - Am - Dm - G - C

The Dm6 is used here as ii-iii, which is acceptable. If you had a Bm7b5, it would be something like:

C - Bm7b5 - C

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    I'd also point out that OP's example is a circle of fifths progression, which usually uses the "Bm7♭5" notation to show the fifth between B and F, and also between E and B. – user45266 May 27 at 22:06
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Yes, popular editions will often write 'Dm6' even when the bass note clearly indicates that it's 'Bm7♭5'. Conversely, you'll sometimes see what is undisputedly 'C6' notated as 'Am7'.

Don't waste time looking for a subtle reason. The editor isn't thinking about harmonic analysis but purely about chord shapes for ukulele or guitar. (The 'Am7 for C6' thing tends to be found in music from the ukulele era, accompanied by 4-string chord shape diagrams. 'Dm6' for 'Bm7♭5' has persisted.)

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D minor6 (Dm6) is spelled D F A B.

D half-diminished (Dm7♭5)is spelled D F A♭ C.

D diminished (Do) is spelled D F A♭ C♭.

So, diatonically, Dm6 works in key C. Dm7♭5 cannot. It may be the weapon of choice in pop songs, but it's not diatonic to the key.

They are not the same chord, don't sound the same, so what's the question?

EDIT - thinking that now, the OP is considering Bm7♭5 and Dm6 both with reference to key C, another thought comes along.

Yes, both contain the same notes (as do C6 and Am7), so context would probably be a deciding factor. Often I find Bm7♭5 followed by Em, whereas Dm6 is followed by G (circle of 4ths/5ths idea). In root position, those sound believable. Using inversions always plays tricks with the ears, so it makes it difficut to pinpoit a 'correct' name. As already stated, it's also the best part of G9 - made up from G B D F A - but missing the important G note can't actually make it G anything!

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    I think you're misunderstanding. The OP is saying that Bm7b5 contains the same notes as Dm6. In the given example, Dm6 is substituting for Bm7b5. – Peter May 25 at 6:55
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TL;DR: it should often be labelled as Dm6/B.

How to label a chord... In discussions around questions like this, there seem to be implied assumptions like the following:

  • (1) for any combination of notes, there exists one way to label it as a chord symbol that's "correct" for all intents and purposes in all imaginable contexts
  • (2) chord symbols in songs should be understood as some kind of harmonic analysis
  • (3) the chord's root note is the be-all and end-all "this changes everything" factor in harmonic meaning
  • (4) in order to know the one-and-only correct functional role and perspective for a chord, you need to identify the one-and-only correct chord symbol label.

I disagree on all of those points, and "Bm7-5" in particular is a prime example. In many cases, instead of Bm7-5 what should really be written is Dm6/B, or even Dm/B or simply Dm. When the chord is used in a simple Am - Dm - E7 - Am song as a fancier Dm, writing "Bm7-5" badly obfuscates the fact that it's doing the job of a Dm. Many players might not even know what the "7" is, let alone the "-5" (sometimes even written in small letters), which is IMO often the most important note in the chord. So they play "Bm" or "Bm7" - with the F# note and all. I know naturally musical people who don't know theory, but can accompany songs by ear, and when hearing this "Bm7-5" chord in a song, they'll play a Dm - which is what the chord really does.

If you write lead sheets for amateur musicians, please stop the "Bm7♭5" nonsense and leave that for theory snobs and more advanced audiences.

Chords are not monolithic objects, even though the practice of using chord symbols might lead you to that conclusion. There's no half-diminished chord receptor in your brain, which would activate only upon hearing a half-diminished chord. Chords consist of separate notes, and each of them can have its own role in the harmonic story. The bass note is not necessarily the most important note in affecting where the harmony seems to turn. Take a song that's in Am and has this "Bm7-5" chord written in it. If you were writing an arrangement of the song for, say, two horns, and if you had to choose just one note from B-D-F-A for a harmony voice, to deliver a barebones version of essentially the same harmonic feeling, which note would you select? Self-evidently the bass note because it changes everything? YMMV, but as a first choice, I'd select either F or D. What if it was a B7 chord? Then the obvious first choice would be D#, not B.

By the way, Thelonius Monk called it "minor six with the six in the bass".

  • It could be argued that in 'Am - Dm - E7 - Am' the Dm is a pre-dominant and thus could be usefully thought of as a watered-down B-rooted chord. – Laurence Payne Jun 30 at 12:06
  • @LaurencePayne Interesting way to put it, thanks. That's in a way how it feels too. On the other hand, Dm also tastes like a mix between what you say, "B-rooted chord" and F major or IV of C major. I guess my point is that all of these tastes, feelings and perspectives can be correct and applicable at the same time. Many different harmonic patterns can be seen in any set of notes. For some reason, students tend to come out of theory classes thinking, "there's only one correct perspective". Anyway, for many people, Am - Dm - E7 is a well-known harmonic pattern they can identify and play by ear. – piiperi Jun 30 at 13:35

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