Image via Wikipedia Learning Altered Chords
The main purpose of altering chords is to increase the effectiveness in a progression.
A dominant seventh is more effective than a dominant triad in 2-5-1 and other chord progressions. Also, learn dominant resolution, playing in all keys with the Circle of Fourths.
The following chart shows various combinations of altered chords:
Maj7 (+5) = A major seventh chord with an added raised fifth tone.
7 (-9) A dominant seventh chord with an added lowered ninth tone.
7 (+9) A dominant seventh chord with an added raised ninth tone.
7 (-5) A dominant seventh chord with an added lowered one-half step.
9 (+5) A dominant ninth chord with an added raised fifth tone.
9 (+11) A dominant ninth chord with an added raised eleventh tone.
11 (-9) A dominant eleventh chord with an added lowered ninth tone.
13 (-9) A dominant thirteenth chord with the ninth tone lowered one-half step.
13 (-9 -5) A dominant thirteenth chord with both the ninth and fifth tones lowered one-half step.
13 (+11) A dominant thirteenth chord with the eleventh tone raised one-half step.
(-9) Add lowered ninth tone.
(-9+5) Add lowered ninth and raised fifth tones.
(-9 -5) Add lowered ninth and fifth tones.
+5 Add raised fifth tone.
+9 Add raised ninth tone.
+11 Add raised eleventh tone.
If you're looking for a study on chords, I recommend Chords 101 and 102
Using Altered Chords in the 2-5-1 Progression
It is common to use altered chords to change the feel of a chord progression. The 2-5-1 progression can be played with minor and major triads, seventh, ninth, eleventh and thirteenth chords. The 2-5-1 progression can also be played with altered seventh chords.
Here is the 2-5-1 chord progression with major, minor, and dominant seventh chords:
D/ DFAC (Dmin7)
G/ GBDF (G7)
C/ CEGB (Cmaj7)
Here is the 2-5-1 chord progression with just dominant and major seventh chords:
D/ DF#AC (D7-root)
G/ DFGB (G7-2nd)
C/ CEGB (Cmaj7-root)
What types of chords do you prefer playing with in a ii-V-I chord progression?
If you're looking for more information on altered chords, using the 7-3-6-2-5-1, you might want to invest in this study course book, 300pg Course Book-Techniques and Fundamentals
Learn the secrets to playing piano by ear!
"The beautiful thing about learning is that no one can take it away from you." B.B.King