While 6th note of Dorian and 4th note of Mixo has special reasons in their avoid note (refer to my workbook page 12), generally avoid note is a note that produces b9th interval against the chord of the moment (this includes anticipation in jazz style). The effect of b9th interval is that it destroys the tonality. As soon as you hear this interval, you looses the sense of unity. This is in human nature. Of course, knowing this, you can intentionally create b9th interval for your creativity. John Scofield does it all the time. The important point of this is you must know what you are doing.
Here is the rule of avoid note you want to repeat until you have it in your body:
- Don’t start with
- Don’t hold with
- Don’t end with
- Don’t jump to/from
Don’t Jump Around Avoid Note
When note moves in the interval more than 2nd, human ear catches it as structure rather than passing note within a phrase. If the last note before the jump or first note after the jump is an avoid note, it stands out and destroys tonality. Slower the tempo, worse the effect.
However, if avoid note is followed by a valid note in half-step, the avoid note will act as an approach note. While it still catches ear, the unity of the tonality won’t be destroyed as bad: