Diminished Chords Theory
There are three common types of diminished chords ("dim" chords). These are - the diminished triad, the diminished seventh, and the half-diminished seventh chords.
Character of the chord
Generally creates a very unstable feel to section of music, due the fifth note being flattened.
Notation
Dim chords are commonly written with a small circle after the letter name. For example, E°. This can also be written Edim.
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E° notated
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Theory - how dim chords are formed.
A dinimished triad is formed from three notes - the root, the minor third (b3rd) and the flattened fifth (b5). Here's how to work one out, if your not familar with these terms:-
C° chord
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Begin by thinking about what any minor chord would look like (In this instance, we're going to think about Cm=C - Eb - G). Next, go down one semitone from the top note of the chord. This gets you to Gb (F#). That's it! Cdim triad is C - Eb - Gb.
However, this is not the end of the story. A diminished seventh chord (as opposed to a diminished triad) also has a diminished seventh note in (a double flat note). This is worked out by taking the last note of the dim triad (in this case, a Gb) and going up three steps (three semitones - remember not to include the first key). This gets you to A (technically Bbb, B "double flat").
So C° is C - Eb - Gb - A
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So if we want to play a C dim triad, this is usually notated Cmb5, or Cm-5. If we are called on to play a diminished seventh chord, this is notated C° or Cdim. However, in some modern jazz notation, C° means play the dim triad, and C°7 means play the diminished seventh chord!
Half-Diminished Seventh Chords
Just to add to the confusion, there's another type of diminished seventh chord! This is the half-diminished seventh chord. These chords are notated with a diagonal line going through the circle, so C half-diminished seventh would look like this - Cø .They are a diminished seventh chord with a seventh that is flattened, not diminished. So Cø would be played C - Eb - Gb - Bb. These half-dim chords are very common in modern jazz, and are used as II chords, in the chord progression II, V, I. An example in C minor would be Dø - G7b9 - Cm. Jazz musicans will often refer to this chord as the "minor seventh flat five" , and notate it Dm7b5 .
TOP TIP:
There are only three different types of diminished seventh chords possible. If we begin on C, there is C° (C - Eb - Gb - A), then Db° (Db - E - G - Bb), and then D° (D - F - Ab - B). However, when we start on the next key up (Eb), and form the Eb° (Eb - Gb - A - C), we are effectively playing the first chord again, in a different inversion. The same happens on the next key up (F, this is the same as the Db°)
If you like making theatrical effects on the piano, try playing the C° as a "tremolo" chord (rapidly playing the notes by rocking the hand from side to side), and then do the same with Db°, and then D°. Hey, now you might get some work accompanying an old black & white movie!
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