An obscure seventh chord in the wild

A month ago, I wrote a blog showing various seventh chords, including a few that are more notional than useful. As it happens, the musical group is working on a song (by my fave Yoni Rechter) than includes one of the rarer seventh chords - C7b5. This chords appears at the end of a line and is then followed by a row of chords descending by a semitone. At first I thought that this was a misprint in the chord chart to be found on a public site (i.e. not that of Rechter) as other songs on this site often have errors in the chords posted, but after thinking it through a great deal, I realised that this indeed is the chord. The melody note played over this chord is F#, and indeed this chord contains an F# (or more accurately, Gb, as it's the G that is being flattened). I tried a few other chords that contain F# but none of them sounded correct.

I saw an interesting YouTube video about a week ago that states as the basis of a school of harmonic thought that all music is based on tritones. For example, the perfect cadence (e.g. G7 -> C) has a tritone in the G7 (B, F) resolving in the C (C, E): that's what makes it sound so perfect. As I tend not to have perfect cadences in my songs, I noted the idea about tritones but didn't think about it very much.

C7b5 is a great example of a double tritone chord: any dominant seventh chord (e.g. C7) is going to contain one tritone (in this case, E, Bb), but this chord has a second tritone (C, Gb). That second tritone would not resolve normally by a semitone if followed by an F chord. The double tritone also makes it difficult to figure out a chord shape for the guitar - one doesn't want the E and the Gb on adjacent strings, and similarly not the Bb and C. In the chord diagram at the top, the shape on the right is probably the one that I will use as I can lower it by semitones as required by the song. The notes, from bottom to top, are C Gb Bb E - this has all four notes, and no two notes are separated by a tone.

After trying out the various shapes, the chord on the left seems the most appropriate. The notes are C, Gb, Bb, E. I actually developed this shape myself before noticing that it was one of the shapes in the picture.

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