Then you see that sus2 is treated the same as add2 and add9. The latter two feature a third while the former, we agree on that, don't.

AFAIK add2 and add9 are the same. Perhaps add2 implies a closed chord with the 2 actually played as a second in comparison to add9 where a voicing of second isn't for me the first choice.

I'm not a Jazzer so I cannot say anything about proper chord additions or substituitions. But I wouldn't except a change within the fifth above the root note. That is, a notated M3, m3, sus2 or sus4 is left alone but may be extended with 7th, 9th flat or sharp etc.

Masi


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