|English: Dominant seventh flat five chord on C (Photo credit: Wikipedia)|
I have learned a great deal of information from Jermaine Grigg's, especially the section on altered chords from his course, The Secrets to Playing Piano By Ear
Introduction to Altering Chords
"Any chord, whether major, minor, augmented or seventh, can be modified or 'altered' thereby changing its character or color. In particular, with the dominant seventh which is mainly characterized by three notes: the root, major third and minor seventh; the fifth, ninth eleventh and thirteenth may be altered.
Raising or lowering the notes of a chord and its extensions by a half step may change its dissonance. This increases the 'tension' of the chord and increases the sense of release as one moves to a less dissonant chord (the tonic). Care must be taken that these altered chords are correctly numbered."
Example: C9 (+11) represents a C ninth chord with the root, a major 3rd, a perfect 5th, a flattened 7th, a major ninth and a sharpened 11th.
Example: C#9 (#11) represents a C# ninth chord with the root, a major 3rd, a perfect 5th, a flattened 7th, a major ninth and a sharpened 11th.
Example: C7 (b9b5) represents a C seventh chord with the root, major 3rd, diminished 5th, minor 7th and minor 9th.
The main purpose of altering chords is to increase the effectiveness in a progression. In previous lessons, we've already learned how a dominant seventh is more effective than a dominant triad in "2-5-1" and other chord progressions.
I recommend to my friends, The Secrets to Playing Piano By Ear
The book goes on to explain using altered chords in the 2-5-1 progression, in the 3-6-2-5-1 progression and the 7-3-6-2-5-1.
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