Occasionally, you may come across a weird-looking chord symbol such as G7b9 or C7b9#5. The b9 and #5 indicate “alterations” to the chord.
Alterations change the “color” of the chord but do not change its character and purpose.
As always, b means to lower a tone by a half-step and # means to raise the tone a half-step.
The chord G7b9 contains the tones of the G7 chord with an added 9th that is lowered a half-step.
The tones of the G7 chord are: G – B – D – F
The 9th from the major scale of G is A, but we still need to flatten it. (Remember that the 9th is the same as the 2nd degree from the scale.)
The final chord is: G – B – D – F – Ab
The chord C7b9#5 also contains a lowered 9 (Db in this case) and its 5th has been raised (to a G#).
That makes the tones for this chord: C – E – G# – Bb – Db
If the chord symbol is 7alt, then you are free to make your own alterations. Usually only the 9th and the 5th are altered, but raising or lowering the 11th and 13th also happens.
Sometimes the alterations are put in parentheses: C7(b9). That is especially helpful on chords that already have a b or # in their name: C#9 is a C# dominant-9 chord, not a C chord with a raised 9!
Occasionally, the symbols - and + are used for b and #. For example: C7-5
That’s it! If you’re already comfortable with building chords from scale degrees then altered chords should not cause you any problems.
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