Diminished 7th chords are symmetric. Each one could be the same quality chord on any root note that is a chord tone inside it. So, is there agreement on which name to use for these chords? I know that a lot of chord charts will just use whatever root note is convenient (or usually not convenient)

I know there's a bunch of ways to use them. When using one as a passing chord, would it make sense to say F-F♯dim7-G in the key of C? Do I call it F♯dim7 because the F♯ is like the leading tone to G? Or something else?

What if I use one in a different way? How do I know which of the notes should be called the root? (And I suppose this question would also apply to augmented triads as well...)

If the diminished 7th can be explained away with the rules of common practice period harmony, the name of the diminished 7th chord should reflect its harmonic function (or Roman numeral notation).

For example, your F-F#°7-G example is correct (in C major) because G is the V chord and F#°7 is easily explained as vii°7/V.

A more exotic example is E°7-A°7-Bbm, which can be explained in Roman numeral notation as vii°7/V into vii°7 into i.

Common-tone diminished 7th chords, such as the diminished 7th chord in a C-C°7-C passage, are harder to name. Naming them such that they share no common tone names with the previous chord (such as Eb°7, which is Eb-Gb-Bbb-Dbb) is a no-no, but you might need to look at the bottommost note in order to determine which name is best.

If you cannot explain the diminished 7th chord with common practice period harmony rules, then assuming the bottommost note is the root is likely the best way to go.

Augmented first. They act in a similar way to dominant chords, thus in a passage C C+ F FM, it would be futile to call the C+ anything else. Yes it contains the same note sounds as E+, and E is diatonic, but the E+ naming would assume the following chord is most likely A.

Diminished chords are a conundrum. I'm sure in Mark Levine's book, and certainly in stuff I've had to play in bands, something such as Ebo has been written with a D#. Follow that for illogic!

However, the function is one criterion, as Dekkadeci says, but the lowest note is another. Having said that, I've played many dim chords where the name is, say, Ebo, and the written bass note is C. There are also examples of the exact same dim. chord written in two different ways in the same song - even more confusing! As always, use of ears is encouraged in the quest to provide best answer!(Although that in itself probably won't help a great deal in some circumstances.)

The main criterion for me would be the bass note, in conjunction with the function of the chord itself. Having said that, when a dim. is played in an ensemble situation, it's usually quite short, and if a different chord tone is more prevalent, chances are not many would spot a 'mistake'. I've even used 7b9 chords instead and no-one has commented...

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