2 and add9 chords
Many times I have had students, even advanced students, tell me they have no idea “what is the difference between these two kinds of chords.” It is not essential to know the difference between a 2 chord and an add9 chord, but it is helpful. The difference between these two chords is not what you might think. It is the third scale tone that makes the difference.
A "2" chord does not have a third. It is spelled 1, 2, 5. (In the key of A this is A, B, E)
An "add9" chord has a third. It is spelled 1, 3, 5, 9. (In the key of A this is A, C#, E, B)
"2" and "9" are the same note. In the key of A, this note is B. Both an A2 and Aadd9 have a B note, but only the Aadd9 chord has a C# note (which is the third). So, it really is the third note that makes the difference.
A "2" chord could always be substituted for an "add9" chord.
An "add9" could be sometimes but not necessarily always be substituted for a "2" chord, because it's possible the "2" chord could be played in a situation in which the major/minor is ambiguous and we would not want to add a Major third in some cases.