I just saw this comment by Tim, in another question:

Do can't have a C. It's spelt D, F, Ab, Cb.Or as a triad, just the 1st 3 notes. Ao/F is F b9, and Ao/F# will just be F#o. Or D b9/F#. If this helps - hope so.

What exactly are , A°/F, A°/F# and F#°? This is the first time I have seen something like this.

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    I assumed that was just a diminished sign in an easier to type form. Technically it should be a superscript degree sign, but that's not easy to input in Stack Exchange. – Pat Muchmore Jan 27 '15 at 15:29

The 'o' bit designates dim, or diminished. Slightly different from the ø which is half-diminished.(Which was pointed out by someone as wrong - it should be a third diminished!) Wish my keyboard could print it properly - it probably can, but I'm not clever enough to make it work! Half-dim.,(that could even be me...) incidentally is aka m7b5.

  • I'd just like to point out that perhaps the oldest known half diminished seventh chord, in this case b d f a, comes in the English medieval canon Sumer is icomen in. – Scott Wallace May 30 '17 at 15:10
  • @ScottWallace - I'm betting it's actually V9 without the root. Can't say I know it. Must listen... – Tim May 30 '17 at 15:15
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    @Tim- yes, do listen to it. But the way the piece is constructed, basically alternate I and ii chords in Ionian, makes the V9 a modern interpolation. – Scott Wallace May 30 '17 at 15:19
  • @ScottWallace - can't find the run mentioned in any of the dots I've trawled through. – Tim May 30 '17 at 15:27
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    @Tim- I guess everything is off subject, if you just look at it hard enough. Yes, with modern ears, it sounds like a g-b-d-f-a ninth chord, minus the root. But the g never happens in the whole piece as root of a chord: this is a good example of an ionian piece that is decidedly not major. – Scott Wallace May 30 '17 at 16:20

D° means Ddim or D diminished. D diminished means D minor with flattened five, i.e., you play D, F, Ab.

I found this site to useful: https://www.pianochord.org/d.html

There is a list of notation explanation. For D°, it says: "Ddim - D diminished (D°)". Then you go to the table above and choose Ddim: https://www.pianochord.org/d-dim.html

Here, it shows you which keys to press.

  • I wonder why in this collection of chords there is no D dim7? D°7? – Albrecht Hügli Jan 24 '19 at 11:59

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