Sharpened subdominant with diminished seventh

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The sharpened subdominant with added diminished seventh chord is something of an obscurity in music theory. It is more simply represented with the Roman notation #iv ^d7. In the key of C, this is F# dim7, which may be used for a strikingly fearful effect, as its root is a tritone (augmented fourth) from the tonic. Alternatively, it is a common chord in jazz and ragtime music. A common traditional jazz or Dixieland progression in Eb may go:

Ab - A dim - Bb7

which is

IV - #iv (^d7) - V7

Another common usage of #IV dim is often found in Gospel music and jazz progressions such as in the song "I Got Rhythm".

In C:

| C C/E | F F#dim7 | C/G A7 | Dmi7 G7 |