If not...

[nyggus]

It was a major, if not the dominant, factor of the collapse.

Should this last comma be there? I think it should but have come across many sentences where it wasn't there. So, should it or not be there?

Thanks,
Nyggus

[Delmobile]

I personally would use a comma here. I don't know if I've ever come across a rule for this situation.

[not a teacher]