An altered chord is usually a dominant chord with one or more changed tensions.
1 3 5 b7
On C7:
That means the base sound is still:
- 1
- major 3
- 5
- b 7
Compared with Cmaj7, the dominant already changes the 7th:
Common altered notes are:
- b9
- #9
- b5
- #5
Example with C7#9:
An altered dominant usually wants to resolve strongly.
Example:
That extra tension is what makes the resolution feel stronger.