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 may start:
- Ab - A dim - Bb7
which is
- IV - #iv (d7) - V7
in the key of Eb, although the 7th is often not included in the second chord.
