Learn to improvise. 2003-2004. Lesson 18: diminished chords.

For improvisation thinking in structure helps: 1) what is the initial key, 2) which are the successive temporary keys, 3) which are the chords.

It is practical to consider the chord structure to consist exclusively of major and minor chords. You can object, that particularly the complex chords (C9+, Cmaj7-5, etc) form the essence of a certain music piece, but still, if you are able to discover a simplified underlying harmonic flow, it will help to keep the thread of the music. Compare the plot of a story.
Together with the primary chords major and minor we also mentioned the diminished and augmented chords. The reason is, they consist of two thirds and are consequently fundamental.

The diminished chords deserve extra attention. If a diminished chord belongs to the basic harmonic flow it is hard to find an acceptable replacement for it among the major and minor chords. An example is the following chord scheme: C Cdim C Cdim C A7 Dm G7...
Hear this scheme.

The name diminished refers to the fifth (Cdim has c to g-flat in stead of c to g). On the one hand it seems logically to talk of C-minor- diminished (Cmdim), as the first interval is a minor third (c to e-flat), on the other hand however one may say both intervals are diminished.

The case is even stranger, because for improvisation Cdim is used as a replacement for Cdim, Ebdim, Gbdim en Adim. Only a connoisseur will be disturbed by the difference. Hear the previous chord scheme, but with Cdim replaced by Cdim7.
Hear previous scheme with dim7 in stead of dim.

In practice it is convenient to have to take into account only 3 different dim7 chords in stead of 12 dim chords. It has become so established, one mostly writes only dim even if dim7 is meant.

The word 'seventh' is strange too, because in C7 the seventh note is b- flat and in Cdim7 it is the a. As an excuse one can count along the diminished scale, which consists of alternately one and two key steps. Then the seventh note is the a. All these remarkable things are also an indication of the application of the diminished chords. Their effect, namely, happens to be a little strange, too.
Typical applications for the diminished seventh chord are the following:

-Nearly every seventh chord may be replaced by the diminished seventh that has 3 of the 4 notes collective. It gives the strange (often interesting) effect, though. It depends on the music style how often you can use it. For bar music e.g. it may be applied much.
Compare scheme Em B7 with Em Ebdim7.

-Interlude during a modulation. Suppose, you have played a song in A, but the singer prefers G. Then you will modulate to G, as soon as the singer starts, but in order make the changing more flowing, you may play Adim7 in between.
Modulation of key A to key G via Adim7.

-For just an interesting effect. E.g. by replacing the chord Cm with Gdim7 in the scheme G G7 C Cm G D7 G of 'When the saints go marching in' (in G). It is incorrect, but funny.
'The Saints' normal and with Gdim7 as a replacement for Cm.

-Emergency chord if you don't know the real one. Often it works well. You have to play something anyway. A dim7 chord may be interpreted as improvised on purpose in stead of incorrect. Moreover, the chance to strike the best fit is 1 out of 3 for a diminished seventh chord, while it is 1 out 24 for the right major or minor chord.

-If it is meant in the composition. E.g. the 2nd chord in 'Stormy Weather' (Scheme: G G#dim7 Am D7).
Hear the effect of the chord dim7.

The real diminished chord with 3 notes is not used much in improvisation. It is too delicate. E.g. the difference between the positions c e-flat g-flat and e-flat g-flat c is more sensitive than between the positions c e-flat g-flat a and e-flat g-flat a c.
The position has more influence for dim than for dim7.

Use dim7 chords. Work playfully with them. Hear the typical atmosphere and get used to it. Play them broken (e.g. like in the Moonlight Serenade of Beethoven) and make melodies with them.
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