How to identify and name Full Diminished Seventh Chords?

Question

I made a frightening discovery -- No mater what inversion diminished seventh chords are in, all of the notes are the same interval apart! A minor 3rd. How then is someone to identify what name the chord has? Here is an example from Jesus Paid it All, a song I'm trying to transpose to F.

The chord in question falls on the 3rd beat in the first measure of the 3/4 section -- over "He". The song, and you can't see this in the image, is in Db major.

So Diminished 7 chord here, with the notes listed top to bottom as displayed here, goes

`Db`
`Enat`
`Bb`
`Gnat`

If I were to rearrange these into a more directly transferrable to the keyboard way,

`Gnat`
`Bb`
`Db`
`Enat`

But I can also rearrange the notes like this:

`Enat`
`Gnat`
`Bb`
`Db`

Or however you please...

So heres the question: How do I name this chord? Using Roman Numeral notation, how would I write it for transposition?

Answer 1

I would write it as Gdim7.

If we take the bass line only from the beginning to the end of the above screenshot (taking into account that the whole tune is in Db major), it is:

| Db Db Db Db Db F | Gb G | Ab Ab Ab Ab | Db |

Then on top of bass G you've got notes Bb-E-Db which is an inversion of the Gdim7 chord.

If we add the chords the harmony can be described as:

| Db Gb/Db Db Db Db Dbmaj7/F | Gb Gdim7 | Db/Ab Ab Db/Ab Ab | Db |

In this case Gdim7 acts as a dominant or a passing diminished chord (#ivo7).

If you think of it, if it was Gb and not G in the bass, then it would be Gb-Bb-E-Db = Gb-Bb-Fb-Db which is exactly Gb7. But our composer decided to add a bit more tension keeping the dominant chord, but lifting the bass by 1/2 tone, which eventually adds more beauty in the music and it seems apparently typical for Gospel style music, as mentioned here: http://iujazztheory.weebly.com/passing-diminished-chords.html

Answer 2

I would call this a vii^o7/V chord because of the function and the context of the surrounding chords. The bass note (a G) leads into a V chord (Ab).

To understand the function of the diminished chord, the first step is to determine what chord(s) are in the penultimate measure, so that we know how the diminished chord resolves. In the final measure, there is a I resolution, and the entire penultimate measure is the V chord. Rather than describe this measure as one beat of I6/4 followed by three beats of V, my inclination is to call the entire measure the V chord--a cadential 6-4 progression.

So the final two bars are V-I. The diminished chord has a G in the bass, and it leads to a V chord. This makes it a vii^o7/V chord, and the progression is IV-vii^o7/V-V-I.

Answer 3

The best way to handle symmetric chords (fully diminished seventh chords or augmented chords) is to check how they are used (good idea with other chords too.) There are two easy to find uses. One is in a sequence (like if one had C7, F7, xx, e7, a7, d7 G7) one could easily guess that the xx is some type of diminished B chord. The other way is to see if the chords acts like some type of dominant ninth with the root missing. For example, B-D-F-Ab may be treated as a G-ninth with the root missing; particularly if preceded by some type of D-minor harmony and followed by some type of C harmony. Another use is just as a bunch of connecting notes with no particular harmonic significance.

Answer 4

You identify what Roman numeral notation label a diminished 7th chord should have through context. People can easily use inversions of diminished 7th chords, use root diminished 7th chords in unusual contexts (common-tone diminished 7ths are particularly evil for this), and use diminished 7th chords to modulate between keys (so a single diminished 7th chord can have multiple valid Roman numeral notation labels!).

In this case, your target diminished 7th chord is immediately preceded by a Gb chord (I'm assuming that the F is a passing tone) and followed by a Db/Ab chord, then a Ab7 chord. Your target diminished 7th chord has a G natural for a root--this is consistent with it being vii°7/V (note that V of Db major is Ab major), and the Db/Ab-Ab7 chord progression (I 6/4-V7 in Roman numeral notation) confirms this. (The Gb--or IV--chord beforehand also supports this, as it indicates that the music hasn't strayed from Db major at this point.) The E natural in the diminished 7th chord is just to make the sheet music easier to sight-read.