diminished seventh

The example illustrates two of the three possible diminished 7th chords.

In music, an interval consisting of a minor seventh which is contracted (diminished) by one semitone, to create an interval of nine semitones (for example A-G♭); alternatively, a chord consisting of a minor seventh which is contracted (diminished) by one semitone. If every voice of the chord is filled in, there is also a minor third and a diminished fifth, for example A-C-E♭-G♭. This chord, which consists only of a series of minor thirds, is ambiguous harmonically and therefore useful in modulation (moving from one key to another).