A
chord consisting of four notes, one more than a
triad, all of which are a
minor third, alternatively, 3
semitones, apart. Generically, the
seventh is the fourth note of the chord, an interval of a seventh above the
root, or
naming-note of the chord.
For example:
- C Eb F# A
- C# E G Bb
- D F Ab B
This is a peculiar chord, generally considered to reside in the minor scale on whose seventh note it sits. It is perfectly symmetrical, unlike the dominant seventh, even more so than the augmented triad; when you invert this chord--place the bottom notes on the top in succession--you lose track of what chord it is.
Normally, the chord is named by the root. This note is conventionally found when the chord is unwound, so that it is read from bottom to top by alternate note names--e.g. C E G B, as a Cmaj7. The intervals between these tones are irregular--4, 3, and 4 semitones respectively; it pulls itself into root position; the second, or one semitone between the B and C in the inversions is the giveaway.
Not so in the case of the diminished seventh, in fact, there are only these three above. Work it out! You will find that these comprise all the notes, in all the possible chords.
Don't get confused, now! I did.
One last note(!): For all of us who have ever watched a grade B, or grade z, science fiction movie, we are familiar with the sound of these chords: entering the mad scientist's laboratory is announced by a succession of them, usually on organ.