Diminished seventh chord

A chord consists of three minor thirds on top of each other (B - D - F - Ab - Cb=B - D and so on). Because of this structure (an endless string of minor thirds), it has no tonal centre and is tonally ambiguous. It is a favourite chord of Romantic composers. It is used for expressive effects or for modulation. When it is based on the leading note, it has three notes in common with the V7b, therefore behaves like a secondary dominant and nicely resolves on the tonic. By semitone lowering of any of the four notes of a diminished seventh and with necessary enharmonic changes, a dominant seventh in a new key is obtained (i.e., a diminished seventh on the leading note of C major or minor turns into a dominant seventh of A, F# or Eb depending on which note moved a semitone downwards. For example, if F is lowered to E, and Ab being equivalent to G#, the new chord E, G#, B, D is the dominant seventh of A major/minor). This is a powerful way to modulate as one diminished seventh may resolve on four different dominant sevenths and they may resolve on four major or four minor tonic chords. See also dominant seventh.